Firefox 39: Find out what is new

Martin Brinkmann
Jul 3, 2015
Updated • May 22, 2018

Mozilla planned to release Firefox 39 on Tuesday but a last minute bug in the application required fixing so that the release has been delayed for later this week. In case you are wondering what this is all about, you find information about the malware-related issue on Bugzilla.

This is one of the cases where you may run into issues when you are downloading builds of Firefox from third-party download sites as you would have downloaded the version with the bug.

After working on the bug for several days, the organization pushed the final version of Firefox 39 to the official ftp server. It usually takes another day or two before the release is announced officially by Mozilla and then distributed to machines running Firefox provided that the browser is configured to update automatically to new versions.

Mozilla pushes out updates for all Firefox channels. This means that Firefox Beta will be upgrade to version 40, Firefox Dev to version 41, and Firefox nightly to version 42 of the browser.

Firefox ESR versions will be upgraded to version 38.1.

The following information provide you with detailed information about the Firefox 39 release.

Firefox 64-bit for Windows is a no-show again. Mozilla has not yet made a decision when it gets released other than stating that it will be in 2015.

Firefox 39 download and update

Firefox is configured to update automatically to new versions. While you can block this from happening, most users will receive update notifications once Mozilla releases the new version officially.

You may check for updates manually at any time with a tap on the Alt-key and the selection of Help > About Firefox from the menu bar a the top.

This opens a small window listing that performs a check for a new version. If it is found, it will be downloaded automatically to the system.

Direct downloads are provided as well if you prefer those. The links below lead to those so that you can download them and run the installer locally afterwards.

  1. Firefox Stable download
  2. Firefox Beta download
  3. Firefox Developer download
  4. Nightly download

Firefox 39 Changes

The release is rather uneventful in terms of new major features or changes.

SafeBrowsing malware detection lookups enabled for downloads (Mac OS X and Linux)

firefox blocked file

This has been enabled on Windows machines for some time. It uses Google SafeBrowsing information to detect if downloads are malicious.

While it protects users from downloading malicious files, it has blocked legitimate files in the past as well.

Linux and Mac users not interested in Safe Browsing can use these instructions to disable the feature.

Share Hello URLs with social networks

hello share link

If you are using Firefox Hello, a real-time chat component that Mozilla built-into Firefox, you know that you have several options to invite others to chat with you.

Mozilla added options to use the Share panel to share these links on social networking sites. It is not really that much of an improvement considering that you had options already to copy links to paste them on social networking sites you are using.

Asynchronous Plugin Initialization

Update: has not landed in Firefox 39 after all.

This is probably the most interesting feature addition for most users. Four of the top 10 chrome hangs in Firefox are related to plugin initialization and instantiation. The top chrome hang happens when plugin-container.exe child processes are created.

The patch modifies the old process by running most of the steps asynchronously. What this means is less wait time (for a step to complete) and thus less hangs as a consequence.

Additional information are provided when you load Bug 998863.

Reserved browser operations in multiprocess Firefox

Web pages may no longer prevent certain keyboard shortcuts in Firefox if it is running in multiprocess mode. The following commands are protected by the browser:

  • New Tab, Close Tab, Next and Prev Tab
  • New Window, Close Windows
  • Quit
  • New Private Browsing Window

Other changes

  • Removed support for insecure SSLv3 for network communications
  • Disable use of RC4 except for temporarily whitelisted hosts
  • The malware detection service for downloads now covers common Mac file types (Bug 1138721)
  • Performance of displaying dashed lines is improved (Mac OS X)(Bug 1123019)
  • Project Silk: Smoother animation and scrolling (Mac OS X) - Additional information here.
  • Support for new Unicode 8.0 skin tone emoi

Developer Changes

  • Support for 'switch' role in ARIA 1.1 (web accessibility)  - click here for ARIA (Accessible Rich Internet Applications) information.
  • WebIDE now supports debugging Firefox OS devices over Wi-Fi (instructions)
  • Drag and drop elements in the Page Inspector (additional information)
  • Web console command history is now persisted across sessions.
  • Fetch API has been activated (bug 1133861)

Firefox for Android

Firefox for Android shares the majority of changes with desktop versions of Firefox.The only Android-specific change lets you paste clipboard contents into editable web contents.

Security updates / fixes

  • MFSA 2015-71 NSS incorrectly permits skipping of ServerKeyExchange
  • MFSA 2015-70 NSS accepts export-length DHE keys with regular DHE cipher suites
  • MFSA 2015-69 Privilege escalation in PDF.js
  • MFSA 2015-68 OS X crash reports may contain entered key press information
  • MFSA 2015-67 Key pinning is ignored when overridable errors are encountered
  • MFSA 2015-66 Vulnerabilities found through code inspection
  • MFSA 2015-65 Use-after-free in workers while using XMLHttpRequest
  • MFSA 2015-64 ECDSA signature validation fails to handle some signatures correctly
  • MFSA 2015-63 Use-after-free in Content Policy due to microtask execution error
  • MFSA 2015-62 Out-of-bound read while computing an oscillator rendering range in Web Audio
  • MFSA 2015-61 Type confusion in Indexed Database Manager
  • MFSA 2015-60 Local files or privileged URLs in pages can be opened into new tabs
  • MFSA 2015-59 Miscellaneous memory safety hazards (rv:39.0 / rv:31.8 / rv:38.1)

Additional information / sources

Now Read: Firefox Release Schedule

Firefox 39: Find out what is new
Article Name
Firefox 39: Find out what is new
Mozilla released Firefox 39, a new version of the web browser. This guide provides you with detailed information about new features and changes.

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  1. juanb said on August 3, 2015 at 11:04 pm

    Hi Andy,

    I’m trying to reproduce your problem. So far no luck. Part of the problem is that I am waiting for one of my machines to be offered the Windows 10 update.

    What kind of account were you using when this happened (administrator vs regular user)? Did you try setting your default web browser in the settings panel before you launch Firefox?

    I’ll keep trying.

  2. Andy said on August 3, 2015 at 9:29 pm

    I updated to windows 10 the first day it came out..Mozilla Firefox 39 has been called Incompatible in my computer (64bit 20 GB ram ). I updated to windows 10 July 29th the Mozilla browser worked until today when this happened. I also updated to Kaspersky Internet Security 2016 today August 3,2015. Turning that off the browser still does not work. I have removed and re downloaded the browser numerous times but still will not work. I wanted Firefox to be my default browser from the start of the upgrade . I have called Kaspersky, asked Windows but to no avail . any ideas ? ( Sorry to change the subject of this venue . )

    1. Caspy7 said on August 3, 2015 at 10:23 pm

      You should be able to file this issue/get assistance at

    2. helper said on August 3, 2015 at 10:19 pm

      hello, you can find the support site for firefox at

  3. Dave said on August 1, 2015 at 3:49 pm

    UnMHT doesn’t work on Firefox 39 so I went back to 38 EME-Free

    1. saad said on August 10, 2015 at 6:13 am

      use Mozilla Archive Format addon

      MAFF and MHT support for your browser, and more.

  4. BrokenEye, True False Prophet said on July 8, 2015 at 11:01 pm

    Damnit, when are they gonna just let Hello die already. Does anybody actually use that bloatware?

  5. Patrick said on July 7, 2015 at 3:49 pm

    Trying to download an MP3 podcast with Firefox 39.0, the software keeps the temporary .part file at the end of the download and drops the file with the MP3 extension. It sounds like an old bug which comes back.

  6. pd said on July 5, 2015 at 4:33 pm

    “Four of the top 10 Chrome hangs in Firefox are related ”

    Critical capitalisation error there? Suspect your meant:

    “Four of the top 10 chrome hangs in Firefox are related “

    1. Robert said on July 5, 2015 at 7:56 pm

      What rule are you based on?

  7. Caspy7 said on July 5, 2015 at 3:22 am

    A Mozilla developer has informed that Asynchronous Plugin Initialization has been disabled for release due to some hangs that were found during 39 beta testing.

    This feature does appear to be removed from the release notes.

    (Also, you may consider un-capitalizing the use of the word chrome.)

  8. Aaron Klotz said on July 5, 2015 at 1:07 am

    Suggested correction: Asynchronous plugin initialization was disabled in 39 during the beta cycle. We found some hangs during 39 beta testing that could not be resolved for the 39 release.

    1. Martin Brinkmann said on July 7, 2015 at 1:39 pm

      Aaron, thanks so much for letting me know. I have updated the article accordingly.

  9. Robert said on July 4, 2015 at 8:53 pm

    I’m very disappointed with the last version of Firefox:
    – My Firefox v37.0.1, minimum memory just opening a blank page: 200MB
    – My Firefox updated to v39 (and with all the new bloatware disabled) minimum memory just opening a blank page: 260MB
    What is taking 60MB? At this rate Firefox will become unusable in old computers soon.

    1. El Goopo said on July 4, 2015 at 11:04 pm

      Perhaps visit about:memory in both, and click “measure”, then compare the difference? Then try again with a new test profile, and see if maybe it’s something in your settings/addons that made the difference (probably not, but once in a while an addon or setting can be the cause).

  10. Dave said on July 4, 2015 at 5:24 pm

    Thank goodness there’s no Pale Moon suggestion in this Firefox article. I was half expecting to read something like:

    “Pale Moon has not felt the need to release such an update. Many users prefer Pale Moon over Firefox and it is available for free from the Pale Moon website. Many Firefox addons will work fine in Pale Moon but if you find one that doesn’t you may want to contact the addon developer and suggest they add support for popular browser.”

    1. El Goopo said on July 4, 2015 at 11:01 pm

      The honeymoon with Pale Moon seems to be ending as they move further and further away from stock Firefox. People are just realizing that all it seriously offers over a 64-bit beta build is the old UI, and with a couple of addons and tweaks you can probably get the new UI working better anyhow. Couple that with Pale Moon breaking addon compatibility and now recently announcing that they’re drifting even further from Firefox’s Gecko engine, there’s precious little reason to use Pale Moon anymore. The placebo effect simply isn’t as appealing as all the under-the-hood fixes and optimizations made to the stock Firefox browser since Pale Moon was forked.

      1. George said on August 10, 2015 at 10:18 am

        Firefox’s UI is horrendous and keeps getting worse. So actually, the reasons to use Pale Moon or other alternatives keep increasing. Now Mozilla is seriously thinking of removing FTP support? Oh, that would be really exhilarating – not to mention a downright insult to users. “Yes, but you can bring it back with an add-on”.

      2. Lestat said on July 8, 2015 at 7:54 pm

        You do now that quite a lot of people do not want to use a Chrome similar Firefox which Mozilla has created in the last few years.

        While Pale Moon has no Australis Add-on compatibility it has something in common with Seamonkey, which i am personally using:

        There are no needs for alternative add-ons to replace Mozilla’s attempt to create a minimalist and simple browser. And quite a lot of people who has left Firefox and now are using Seamonkey or Pale Moon have left because of that.

        If Mozilla would not have felt the need to go in direct competition with Chrome to get Chrome users they never would have felt for the need to make Firefox more attractive for these kind of users.

        What Mozilla has done with Firefox is a true shame. Firefox has gone downhill in rating numbers… and a big part of that user loss is because people want Firefox to be Firefox and no cheap combination of “Best of Chrome elements”.

  11. Guest said on July 4, 2015 at 1:06 pm

    I don’t get all this hype for 64-bit Firefox. I tried Cyberfox 64-bit and there is pretty much zero difference in speed etc. over 32-bit Firefox.

    e10s Nightly on the other hand, now THAT makes a huge difference to speed. Multi-process is where it’s at.

    1. El Goopo said on July 4, 2015 at 10:59 pm

      That’s why 64-bit wasn’t as high a priority as the multiprocess stuff. It does offer a few minor security features and in some cases a slight performance benefit, but the biggest help is that it gives the browser process access to more RAM. However, the work Mozilla have done toward making Firefox multiprocess has solved more of these problems and should also help make the memory limitations less problematic as well, so 64 bit on Windows has been a long time coming.

  12. Frank Black said on July 4, 2015 at 12:20 am

    Still like Pale Moon better.

    1. DonGateley said on July 4, 2015 at 3:32 am

      Thanks for sharing.

  13. v said on July 3, 2015 at 9:09 pm

    Every time I update Firefox, I open about:config and search for https:// and http:// and set most of them to empty. Firefox by default loves to use Google and Yahoo services, I do not.

    1. Pants said on July 4, 2015 at 6:57 am

      You can do that in a user.js in the profile directory. example: create a new text file, rename it user.js – open in a text editor of your choice – add stuff – like this

      // disable tracking protection
      user_pref(“privacy.trackingprotection.enabled”, false);
      user_pref(“services.sync.prefs.sync.privacy.trackingprotection.enabled”, false);
      user_pref(“browser.polaris.enabled”, false);
      user_pref(“browser.trackingprotection.gethashURL”, “”);
      user_pref(“browser.trackingprotection.getupdateURL”, “”);
      user_pref(“privacy.trackingprotection.pbmode.enabled”, false);
      // disable link prefetching
      user_pref(“network.prefetch-next”, false);
      // disable dns prefetching
      user_pref(“network.dns.disablePrefetch”, true);
      user_pref(“network.dns.disablePrefetchFromHTTPS”, true);
      // disable seer/necko
      user_pref(“network.predictor.enabled”, false);
      // disable search suggestions
      user_pref(“”, false);
      // disable link-mouseover opening connection to linked server
      user_pref(“network.http.speculative-parallel-limit”, 0);


      Stick the user.js in your profile directory. Everytime you start FF, preferences from the user.js will override those in about:config. You can change those in about:config (eg for testing), but next FF start everything will revert to user.js prefs.

      1. PPete said on July 16, 2015 at 10:30 pm

        Thanks Pants, you present a nice neat way of doing it + comments (didnt know you could do this in there, in fact i never cared to notice that its javascript) :)
        nicely spotted, its like these things get moved around to evade us.

      2. Pants said on July 5, 2015 at 7:50 am

        also … you don’t put in the [code] and [/code] bits – I only added that on here as some sites will wrap the unaltered contents into a selectable section

      3. Pants said on July 5, 2015 at 3:01 am

        @Pete It was an example – there are a lot more settings for privacy / tracking / security. Don’t use my example as a definitive answer.

      4. Pete said on July 4, 2015 at 5:16 pm

        and this will disable tracking protection?

  14. DonGateley said on July 3, 2015 at 8:39 pm

    If one is running a stable version and installs the dev version does it replace the stable version or install it under it’s own path so one can have both available? Thanks.

    1. Martin Brinkmann said on July 3, 2015 at 9:23 pm

      You can run them separately, but they may share profiles. My suggestion is to create a new profile for the new version and use it. Check out this article for detailed instructions:

      1. DonGateley said on July 4, 2015 at 8:46 am

        What about colliding at C:\Users\~~~\AppData\Roaming\Mozilla\Firefox ? (Or perhaps elsewhere as well, such as where plugins are stored outside of that structure.)

        Do I just need to make sure they use distinct profiles in it? Or is there a .js parameter somewhere that can point the dev version to a different AppData subdirectory?

      2. Martin Brinkmann said on July 4, 2015 at 10:30 am

        You install the Dev version in a different directory, create a different profile for it and link those together. That’s it.

        The directory you mentioned holds the profiles which separate the data.

      3. DonGateley said on July 4, 2015 at 3:00 am

        Thanks Martin. “firefox.exe -profilemanager” is in all my FF shortcuts so I’m familiar with profiles and their use. What I’m wondering about is the executables and the directories that support them. If I install the dev version will my stable version be replaced or will they exist side by side?

      4. Martin Brinkmann said on July 4, 2015 at 7:06 am

        You can run all Firefox channels side by side, no problem. Just make sure you install them in different directories.

  15. Gordon said on July 3, 2015 at 4:36 pm

    Today I downloaded firefox 64 bit Beta version 39 it works a dream a long wait but well done firefox

  16. Pete said on July 3, 2015 at 3:13 pm

    Firefox releases page doesn’t list the new version:

    Mozilla web sites are unreliable, same thing happened with Thunderbird releases page.

  17. Inderjeet said on July 3, 2015 at 2:00 pm

    I like the SafeBrowsing malware
    detection feature. Its important to inform whether the software is dangerous or not. Nice post martin. Thanks for sharing

  18. Daniela said on July 3, 2015 at 1:00 pm

    I liked the “Asynchronous Plugin Initialization” one. Even I’m not using any plugins anymore (just using the OpenH264 Video Codec),but I have friends that do use Flash Player, so this make me happy :)

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