Firefox upcoming WebExtension APIs revealed

Martin Brinkmann
Jun 25, 2018
Updated • Jun 25, 2018

When Mozilla released Firefox 57 back in November 2017, support for the new WebExtensions system for browser extensions was limited.

Lack of support for certain APIs was a big problem as it meant that some extension developers could not port their extensions to the new extension system at all, had to omit features to do so, or wait for APIs to become available to produce a port.

The issue would not have been that problematic if Mozilla would not have made the decision to remove the old add-on system in Firefox 57.

New WebExtension APIs were introduced in newer versions of Firefox and others were improved to include new or missing functionality.

While it is fair to say that the WebExtensions system willl never support the same capabilities that Firefox's classic add-on system supported, it is clear that Mozilla is making progress and that the new extension system has become more powerful since the release of Firefox 57.

firefox webextensions apis

Mozilla revealed a list of WebExtension APIs that it plans to include in coming versions of the Firefox browser.

Starting with the release of Firefox 62, the following WebExtension APIs will be introduced in Firefox:

API target release
userScripts 63
topSites 62
desktopCapture (TBD) 63
declarativeContent 63
Session management 63 (TBD)
Toolbars 63 (TBD)
Overlays 64 (TBD)

There is also discussion going on about introducing a color filter API. Mozilla did not reveal any other information about the upcoming APIs but the API names hint at what those could provide.

Note that the following is my best guess based on the name and some information provided by Bugzilla and Mozilla Wiki pages.

  • userScripts -- management of userScripts and support for running userscripts in sandboxes and make userscripts more reliable See bug 1437098 and Wiki.
  • topSites -- extend the functionality of the topSites WebExtensions API. See bug 1446915
  • desktopCapture -- implementation of Chrome's chrome.desktopCapture API. See bug 1303919
  • declarativeContent -- to match Chrome's declarative Content API. See bug 1435864.
  • Session Management -- give extensions control over sessions. See bug 833791
  • Toolbars -- unclear, maybe control over existing toolbars and options to move icons and such?
  • Overlays -- secure overlay API to inject without "spamming the website DOM". See bug 1340930.

Some APIs add much requested functionality; session management is probably the number one here as extensions cannot control the functionality directly in Firefox currently. Extensions like Session Boss help themselves by using their own load and save functions.

Closing Words

The new APIs do extend the capabilities of Firefox's extensions system further, and that is a good thing. I'm looking forward to userScripts, session management, toolbars and overlays, as they will improve functionality signifcantly (not sure about toolbars yet).

Now You: What is your take on the additions?

Firefox upcoming WebExtension APIs revealed
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Firefox upcoming WebExtension APIs revealed
Mozilla revealed a list of WebExtension APIs that it plans to include in coming versions of the Firefox browser.
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  1. Axel Luessow said on June 27, 2018 at 10:46 am

    It’s probably not too little, too late – but Mozilla seems to have lost a lot of good faith with some decisions including mandatory code signing, telemetry enforcement and dropping legacy extensions before webextension apis were ready. Plus integration of screenshots, pocket and sync is happily putting private data into the cloud.

    All in the name of performance, stabilty, security and convenience – making Mozilla sound like a clone of Microsoft or Apple, i.e. protecting their ecosystem and protecting the users from themselves. And like Microsoft, Mozilla didn’t shy away from simply dropping a programing model like Jetpack/sdk when it suited them – Silverlight, anyone?

    Pity is, the original motivations were perfectly reasonable – rogue local addons, outdated plugins, incompatible overlay extensions, pityful multi-core performance and glacial development speed.

    If Mozilla would have put the announcement in the Quantum hype release “We’ll extend the api until almost all wishes are satisfied” it would have made a big difference. But they simply went for the most popular addons, completely ignoring that Firefox cannot only compete as a Chrome clone. The legacy since Phoenix is about making the browser do what the user wants – which includes space for experiments and specialized addons.

    Btw “In the next six months, we anticipate landing WebExtensions APIs for clipboard support, bookmarks and session management (including bookmark tags and further expansions of the theming API).”

  2. Anonymous said on June 25, 2018 at 4:35 pm

    Well I will be brief but as far as I am concerned it is time firefox just died. I don’t know who is in control of this garbage today but it is nothing like it used to be and has out lived it’s usefullness.They try to push this junk over and over but in this day and age another chrome wanna be we don’t need. The world between phone and computers is so vast it is becoming very hard to find a browser for a computer. The phone world (tablets) is for those that don’t care about privacy or rights. We are in a changeing world run by people that truly are a brick short of a load. Enough rambling though let’s just let firefox fade into the abyss and the world of computers will be a better place for it.

    Good luck and stay safe

    1. yogaisevil said on June 26, 2018 at 2:03 am


      Can you make an article on your users: traffic, origin, browser usage, etc. I’m curious how your demographics compares to something globally like statcounter.

      1. Martin Brinkmann said on June 26, 2018 at 6:42 am

        We don’t collect these kind of information ;)

    2. Richard Allen said on June 25, 2018 at 10:45 pm

      “as far as I am concerned it is time firefox just died.”

      Okay. What browser are you using? Exactly how is it better? And convince my why I should use it instead of Firefox.

      1. Rick A. said on March 31, 2019 at 1:39 am

        “Okay. What browser are you using? Exactly how is it better? And convince my why I should use it instead of Firefox.” – 9 months later and no reply, lol, what a joke.

  3. Paul(us) said on June 25, 2018 at 2:43 pm

    Loving the fact that user-scripts are coming back to Firefox town again.
    But how about Greasmonkey (Ghacks his last article is from 2016 and the last update was from last year) or Tampermonkey (Ghacks his last article is from 2016 and last update from last year)?
    So what to use maybe Greasemonkey, tampermonkey or User-scripts?

    Maybe you Martin could give us a new tour in the land of scripts. How to search fast to the script somebody needs, explanations for the beginner, warning’s from what not to do for the beginners, ferry handy links to great userscript, etc. ? Maybe even a sort of a go-to article what to do?
    Could this be possible it would be from main side ferry appreciated?
    I am asking this because I still have the feeling that I am doing something wrong, especially with Google Chrome I always keep error statements cropping up.

    But how do Martin (Or anybody else) are thinking about the webExtension: Custom Style Script (Inject desired CSS or JS)

    Maybe I read like an old man who is saying that everything uses to be much better but this is only the case when it concern Tab mix plus which I have tried as a WebExtension but was not working properly. Hopefully, there will be one as good or maybe better tab manager. Is there any site of that happening in the future maybe even near future?

    1. Martin Brinkmann said on June 25, 2018 at 3:51 pm

      I don’t know the entire functionality of Mozilla’s implementation but it seems that extensions like Greasemonkey will be able to make use of it to improve things in several important ways.

      Extensions may start supporting the new functionality as early as Firefox 63 if it is indeed launched alongside that version of the browser.

      I don’t really use userscripts all that much anymore but Greasy Fork and OpenUserJS are probably the number one repositories for them right now: and

      A good starting point is the beginners howto here:

      As far as replacements for Tab Mix Plus are concerned, I don’t know if the entire functionality will ever be replicated by a WebExtension. You could follow development of the WebExtensions version of the add-on as it is in active development:

  4. John said on June 25, 2018 at 2:28 pm

    Given that Firefox’s market is still shrinking I think any additional API improvements can’t hurt. But as it swims in the muck with Edge browser I don’t see it ever recovering at this point. Its a pretty decent browser right now, although I still find it has issues with some sites and battery life in my experience is the worst with Firefox. I was really hoping Firefox could stick it out and not allow Chrome to run away with it. But clearly that is happening and I don’t see any browser right now that will catch up to Chrome in market share. Its just not going to happen at this point.

  5. Sören Hentzschel said on June 25, 2018 at 1:35 pm

    > Toolbars — unclear, maybe control over existing toolbars and options to move icons and such?

    it’s about adding new toolbars

    btw Martin, the text “Check the box to consent…” should really be a label. It’s very bad UX that it’s not clickable. Could you fix this please? :)

  6. AnorKnee Merce said on June 25, 2018 at 1:20 pm

    Mozilla is probably doing this only because Firefox has been losing world marketshare(= below 10% in May 2018) since FF57’s Webext in Nov 2017 = trying to ape the success of Chrome and its Webext.

    “Too little too late”.?

    Imagine Ubuntu trying to ape Win 7 or Win 8.1 or Win 10, eg forced auto-updates/upgrades, forced Telemetry & Data collection, cumulative updates/Patch Rollups, processor-blocking updates, Metro tiles/Unity desktop, etc.

    1. Anonymous said on June 26, 2018 at 5:37 am

      I think there’s no too little too late in browser world. If you noticed Edge has been gaining user because it’s fast like what Chrome was.

      Firefox is as bloated as Chrome with the same interface and functional capability as Chrome, so what’s the advantage using Firefox over Chrome? For now there’s none.

      If Firefox can offer advantages over Chrome, people would be using Firefox again which quite unlikely will happen.

    2. yogaisevil said on June 26, 2018 at 1:33 am

      That diatribe against Mozilla is unfounded and stupid. Nerd.

      Imagine Anorknee Merce trying to ape actual apes because he can not function around or coexists with humans outside of the internet.

  7. Franck said on June 25, 2018 at 1:10 pm

    Great news !

  8. Ghacks should be this said on June 25, 2018 at 1:04 pm

    Mozilla if you’re reading this please don’t forget about this badly needed improvement to the webRequest API Many extensions such as Decentraleyes will need it to fix some problems

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