Firefox 42: Find out what is new

Martin Brinkmann
Nov 2, 2015
Updated • Dec 16, 2015

Firefox 42 will be unveiled officially on November 3, 2015 by Mozilla. This guide walks you through the core changes and feature editions of Firefox 42 for the desktop and Android.

All other Firefox channels will receive updates as well on this date. In particular, this means that Firefox Beta will be updated to Firefox Beta 43, Firefox Developer Edition to version 44, Firefox Nightly to version 45, and Firefox ESR to version 38.4.

The information you find below covers only Firefox 42 and no version that is still in development. I suggest you check out our Firefox category here on Ghacks if you are interested in keeping an eye on future changes as we cover those regularly here.

Firefox 42 is a big update that brings along with it notable new features and the long awaited 64-bit version of the browser for Windows.

Firefox 42 download and update

If you are running Firefox already you may use the internal update checker to download and install the new version. Please note that updates may not yet be available depending on when you are reading this post.

To check for updates tap on the Alt-key on your keyboard, and select Help > About from the menu that opens up.

This displays the current version of the web browser and whether an update is available. If it is, it may either be downloaded and installed automatically or manually depending on the update settings of the browser.

If you are new or want to download an installer instead, use the following links to do just that.

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

Special Case: Firefox 32-bit to 64-bit

firefox 64bit
Firefox about:buildconfig page

You cannot upgrade a 32-bit version of Firefox to 64-bit directly. What you need to do instead is download and install the 64-bit version, and have it use the Firefox profile that you are using in the 32-bit version so that you don't lose any data in the process.

I suggest that you create a backup of the profile before you do that though to be on the safe side.

How do you know if you are running a 64-bit version of Firefox? Load about:buildconfig in the browser's address bar and check the "target" string under build platform. If you see "64 in the string you are running a 64-bit version of the browser.

Firefox 42 Changes

The introduction of 64-bit stable builds are not the only important change in Firefox 42.

Private Browsing with Tracking Protection

Originally designed to come out with Firefox 39, Tracking Protection is finally launching in this version of the web browser.

It is an add-on for the browser's private browsing mode that blocks a selection of web trackers automatically in that mode.

To enable or disable it, load about:preferences#privacy in the browser's address bar and check or uncheck the "Use Tracking Protection in Private Windows" box on the Privacy page once it opens.

The effect? A comparison of the loading time of the top 200 Alexa websites came to the conclusion that load time dropped by 44% on average with Tracking Protection enabled. In addition, it reduced the average data usage by 39% and the number of HTTP cookies set on the system by 67.5%.

New Control Center with site security and privacy controls

new control center

A new shield icon is displayed under certain conditions in regular browsing and private browsing mode.

In regular mode, it will tell you if the connection to the site is secure or not. In private browsing mode, information about trackers found on the page and controls to disable the Tracking Protection feature are added.

Audio indicators and one-click audio muting

Audio indicators and muting options are finally available in all versions of Firefox. The browser indicates if audio is playing in any of the tabs open in it with an icon that it adds to the right side of the page title.

You may click on the icon to toggle the audio at any time to either mute the audio or unmute it. Please note that this won't affect playback of the media content on the page in any way though.

This can be very useful to indicate where audio is coming from, especially if it starts playing in the background, for instance when triggered by advertisement or an auto-playing video on a page.

Other changes

  • Improved performance on interactive websites that trigger a lot of restyles
  • Login Manager improvements: improvements to saving usernames and passwords, options to edit and show all logins in line, and migration of passwords from Chrome and Internet Explorer on Windows.
  • Media Source Extension support is available on all sites visited in the web browser.
  • WebRTC improvements: IPv6 support among other things.

Developer Changes

Firefox for Android

Most changes made to the Android version are identical to those of the desktop version. This is true for Tracking Protection in private browsing for example or under the hood improvements.

The changes listed below are exclusive to Firefox 42 for Android.

Android changes

  1. External urls (for instance from chat applications or email apps), can be opened in the background now in Firefox for Android.
  2. Family friendly browsing supported when restricted profiles are created on tablets.
  3. Voice input for searching from the URL bar supported.

Security updates / fixes

Mozilla reveals security patches after the official release. We will update the review as soon as they become available.

  • 2015-133 NSS and NSPR memory corruption issues
  • 2015-132 Mixed content WebSocket policy bypass through workers
  • 2015-131 Vulnerabilities found through code inspection
  • 2015-130 JavaScript garbage collection crash with Java applet
  • 2015-129 Certain escaped characters in host of Location-header are being treated as non-escaped
  • 2015-128 Memory corruption in libjar through zip files
  • 2015-127 CORS preflight is bypassed when non-standard Content-Type headers are received
  • 2015-126 Crash when accessing HTML tables with accessibility tools on OS X
  • 2015-125 XSS attack through intents on Firefox for Android
  • 2015-124 Android intents can be used on Firefox for Android to open privileged files
  • 2015-123 Buffer overflow during image interactions in canvas
  • 2015-122 Trailing whitespace in IP address hostnames can bypass same-origin policy
  • 2015-121 disabling scripts in Add-on SDK panels has no effect
  • 2015-120 Reading sensitive profile files through local HTML file on Android
  • 2015-119 Firefox for Android addressbar can be removed after fullscreen mode
  • 2015-118 CSP bypass due to permissive Reader mode whitelist
  • 2015-117 Information disclosure through NTLM authentication
  • 2015-116 Miscellaneous memory safety hazards (rv:42.0 / rv:38.4)
  • MFSA 2015-115 Cross-origin restriction bypass using Fetch

Additional information / sources

Firefox 42: Find out what is new
Article Name
Firefox 42: Find out what is new
A thorough look at the new features and changes in Mozilla's Firefox 42 Stable release.

Tutorials & Tips

Previous Post: «
Next Post: «


  1. san said on November 21, 2015 at 3:24 pm

    I’m on Win 10 x64 and FF 42.0 64-bit has been far more stable than 32-bit since I can now cross over 3~4GB memory usage without freezing or hickups, and Session Manager is far more reliable now (previously it would randomly get corrupt/unreadable and unable to save a session when memory usage gets higher than 1.8~2GB with too many tabs open).

  2. Anonymous said on November 13, 2015 at 5:16 pm

    Mega download error ” out of memory”.
    Help Please

  3. Dyle said on November 9, 2015 at 5:22 pm

    Anyone knows how to disable this mute button?
    Sometimes I accidentally click the button instead of selecting a tab.

    1. Martin Brinkmann said on November 9, 2015 at 8:21 pm

      I only know of an option to hide the audio indicator.

  4. Tom said on November 7, 2015 at 4:04 am

    I tried installing the 64 bit but not sure if I did it right. I uninstalled previous version of FF first and then installed latest version 42. I went to about:buildconfig and this is what I found, target i686-pc-mingw32 Can you tell me how to install the the 64 bit version? Thanks.

    1. san said on November 21, 2015 at 3:12 pm

      about:buildconfig should show this string:
      (I’m on Win10 x64)

      You have to make sure you’re getting the 64-bit installer, as it’s still not made public on the regular download page, but you can get it here:

  5. 64bit User said on November 6, 2015 at 2:06 pm

    I installed the 64bit, so far seems faster for me but time will tell.

    How do i delete the 32bit without deleting my bookmarks, addons, preferences etc.?

    1. XenoSilvano said on November 11, 2015 at 12:39 am

      just delete it

  6. Martin_S said on November 5, 2015 at 6:31 pm

    Why java does not work on version 42?
    When the applet starts the browser freezes …

  7. StefanW said on November 5, 2015 at 4:50 pm

    The cookie handling has changed, the exceptions are now divided in http and https.
    I have a fairly lengthy list of sites that I allow to set cookies and many of them stopped working.
    Is there a way to get the old settings back? To get rid of the http and https in exceptions that is.


  8. Peter said on November 5, 2015 at 12:07 pm

    Thanks Martin excellent help, working now.

    (In ConFig Fox I switched off all 4 sub options under Disable Tracking Protection)

  9. Anonymous said on November 5, 2015 at 12:02 pm

    Yes. Thanks for your helpMartin all ok now.
    I turned off all 4 Sub options.

  10. Peter said on November 5, 2015 at 11:25 am

    Under options I turn on but next use its switched itself off?

    Not Sandboxie

    1. Martin Brinkmann said on November 5, 2015 at 11:34 am

      Check Config Fox. Tracking Protection may be disabled by it.

      // 0420: disable tracking protection – //
      // I believe there are no privacy concerns here, but you are better off using an extension such as uBlock Origin

  11. Peter said on November 5, 2015 at 8:38 am

    Hi Martin
    Is there a about:config setting yo keep Tracking protection permanently on whilst in Private mode ?

    1. Martin Brinkmann said on November 5, 2015 at 9:11 am

      Not sure I understand what you mean, it is turned on automatically by default.

      1. Peter said on November 5, 2015 at 11:09 am

        Not on my Computer.
        Could be Sandboxie or ConfigFox setting?
        Will have try to find out.

      2. Martin Brinkmann said on November 5, 2015 at 11:10 am

        Peter, check under Options > Privacy. Is it enabled there?

  12. XenoSilvano said on November 4, 2015 at 6:45 pm

    I was looking for the 64bit version of Firefox for Windows on [ ]

    I could not find a download button. I hovered over the download buttons for both the 32bit and 64bit version of Firefox for Linux to reveal the url to see what the discrepancy between the two was:-
    [ ]
    [ ]

    Besed on the url formate for Firefox 64 for Linux I come up with the following based on the format above:


    64bit version of Firefox can be identified through the Task Manger (ctrl+alt+del)

  13. Doc said on November 4, 2015 at 5:54 pm

    For those who are wondering, the switch to revert the “view source” option (in about:config) is “” (change to “false” to view source in a popup window, which I prefer to view source and rendered HTML side-by-side).

    1. Anonymous said on November 12, 2015 at 4:00 pm


  14. John said on November 4, 2015 at 10:08 am

    They removed the reading list from Firefox which I am really disappointed about. I had to revert back to Firefox 41 just to get them. It was a great feature and super useful.

    1. Anonymous said on April 25, 2016 at 5:02 pm


  15. richter said on November 4, 2015 at 4:34 am

    I download extremely large pages with unMHT (think news articles with 1000~10,000 comments) and i am pleased to report that Firefox x64 can handle the upper echelon of that task (pages in the 10,000 comment range) without crashing; something x86 couldn’t do because unMHT would always run out of memory.

    I am a very happy customer right now.

  16. Bud said on November 4, 2015 at 2:34 am

    I installed Firefox 64bit but I’m missing a lot of plugins which were present in the 32bit version. These include: Google Talk, Google Talk Video Renderer, Quicktime, VLC, Java, Silverlight, Microsoft Office 2010, Intel Identity Protection Technology, Nitro PDF, and PDF-Xchange Viewer. I’m guessing they’re either not supported yet for Firefox 64 or the 64bit versions, if they exist, have to be downloaded and installed?

    I went back to FF 32bit in the meantime.

    1. Martin Brinkmann said on November 4, 2015 at 7:53 am

      Firefox 64-bit will limit plugin access. IIRC only Flash will be supported.

  17. AJ North said on November 3, 2015 at 8:42 pm

    Hello Martin,

    Have you any knowledge as to whether MozBackup (a 32-bit app, of course) might be able to migrate all the content of a 32-bit FF installation to the 64-bit version (or whether that utility’s vendor might be planning to upgrade MozBackup if it can not)?


    With regards,


    1. Martin Brinkmann said on November 3, 2015 at 8:44 pm

      I don’t think that is necessary at all since profiles contain all the relevant data and they are used by 32-bit and 64-bit versions alike as far as I know. So, in theory, this should not impact Mozbackup at all.

  18. firefox said on November 3, 2015 at 7:11 pm

    Thanks, I wonder why not.

  19. firefox said on November 3, 2015 at 6:10 pm

    trivial, but anyone know why the 64 bit link not on the main page yet

    1. Martin Brinkmann said on November 3, 2015 at 7:03 pm

      I think they mentioned somewhere that they won’t advertise it the first time.

  20. fokka said on November 3, 2015 at 2:53 am

    oh my god, it’s happening! 64bit on stable, the future has arrived! now we only have to wait for e10s and whatever the move away from XUL will bring us…

    i know i’m sounding a bit cynical, but i’m actually really glad we’re finally 64bit on stable and i’m looking forward to the other changes as well.

    1. Ben said on November 3, 2015 at 4:49 pm

      > and whatever the move away from XUL will bring us…
      The death of firefox :)

  21. wildw1ng said on November 2, 2015 at 8:22 pm
    1. Jeff said on November 3, 2015 at 2:25 pm

      Here is the link to all the 42.0 (release) downloads:

  22. Dwight Stegall said on November 2, 2015 at 6:36 pm

    I started using it more and found a lot of pages either going blank as I scroll down them or not loading at all. They still have some work to do on it yet. Also Social Fixer addon keeps forgetting it’s settings. Setting it up again only lasts a day or two before it does it again. I’ll be staying with Chrome as my default browser a bit longer.

  23. Tom Hawack said on November 2, 2015 at 6:18 pm

    Double your pleasure double your fun : updated to Firefox 42 64-BIT. I notice no speed enhancement at this time.

    Adobe Flash not concerned since it was removed two months ago. I had only one plugin which vanished since it is a 32-BIT one.

    about:options in its window disappeared to be replaced systematically with about:preferences in-content : seems to be a Mozilla obsession to integrate the options panel in-content. Would be smarter to forget the Options and apply the in-content to the library which requires it. Wonder what the average Mozilla development team IQ is.

    Oh! new as well the tab spotting the audio source. Wow, this is gonna change my surf experience. Oh brother …

    Firefox 42 64-BIT doubles my fun, mu pleasure much less. 4Gigs of RAM here makes me unsure to keep the 64-BIT build. I see no advantages and I lose a 32-BIT plugin.

    1. Pants said on November 2, 2015 at 7:41 pm

      Double? Bit more than that … technically speaking … 32-bit systems can address/point to 4 gigabytes (GB) of memory in one go. 64-bit systems can potentially address up to 4 billion times that many locations

      1. Tom Hawack said on November 2, 2015 at 8:24 pm

        I was only playing on a tv commercial from the sixties for DoubleMint Gum I think it was :) …
        Of course 64-BIT applications develop their full potential with 4GB < RAM <= 64GB. As I mentioned above here with my 4GB of RAM a 64-BIT browser doesn't bring me enough to loose the 32-BIT advantages should they be only the ability to run 32-BIT plugins. Strange enough LibreOffice 64-BIT seems to run much better/faster than its 32-BIT version when this seems not be the case with Firefox. So, I've switched back to Firefox 42 32-BIT here on my Win7 64-BIT …

  24. wybo said on November 2, 2015 at 5:57 pm

    I would be interested in the 64 bit version. But how do I transfer my profile. Keeping in mind that I have several profiles.

    Thanks for any advice.

    1. Martin Brinkmann said on November 2, 2015 at 6:00 pm

      You don’t transfer profiles. The 64-bit version uses the same profiles as the 32-bit version.

  25. Joker said on November 2, 2015 at 5:55 pm

    “You may revert the change however.”
    For how long…

  26. anohana said on November 2, 2015 at 5:54 pm

    Is it able to use tracking protection in normal mode (non-private)?

    1. Martin Brinkmann said on November 2, 2015 at 5:58 pm

      Yes it seems possible. Just set privacy.trackingprotection.enabled to true on about:config.

  27. Bobbyc said on November 2, 2015 at 5:32 pm

    It seems the 64-bit version doesn’t play nice with flash, lots of black video.

    1. Chains The Bounty Hunter said on November 4, 2015 at 11:51 am

      I’m using the 32-bit build and just ran into the same issue with HTML5 video, so I’m guessing this is some unforeseen bug they’ll be addressing with a patch shortly.

    2. riti said on November 3, 2015 at 9:15 pm

      why not html5

      1. Doc said on November 4, 2015 at 5:51 pm

        Not all sites have enabled HTML5 video; for example, still uses Flash to play videos.

    3. A&L said on November 3, 2015 at 4:02 pm

      I just tried the x64 and the first embedded youtube video i came across wouldn’t play, tried viewing it on youtube and still wouldn’t play
      back to cyberfox 64

      1. A&L said on November 6, 2015 at 6:29 pm

        removed FF 32 an 64, deleted profile folder after backing up with mozbackup, installed 64 and videos now play
        also exported bookmarks, wrote down saved passwords and addons

Leave a Reply

Check the box to consent to your data being stored in line with the guidelines set out in our privacy policy

We love comments and welcome thoughtful and civilized discussion. Rudeness and personal attacks will not be tolerated. Please stay on-topic.
Please note that your comment may not appear immediately after you post it.