Add-on Compatibility for Firefox 51

Firefox 51 will be released on January 24th. Note that the scheduled release on December 13th is a point release, not a major release, hence the much longer cycle. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 51 for Developers, so you should also give it a look.


XPCOM and Modules


  • Embedded WebExtensions. You can now embed a WebExtension into any restartless add-on, allowing you to gradually migrate your code to the new platform, or transition any data you store to a format that works with WebExtensions.
  • WebExtension Experiments. This is a mechanism that allows us (and you!) to prototype new WebExtensions APIs.

Let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 51, I’d like to know.

The automatic compatibility validation and upgrade for add-ons on AMO will happen in a few weeks, so keep an eye on your email if you have an add-on listed on our site with its compatibility set to Firefox 50.

13 comments on “Add-on Compatibility for Firefox 51”

  1. john wrote on

    firefox has been saying since 46.0 that web extentions are here bull even a google search has no web extentions firefox also said there would be a seperate web ext section in the addons nothing there either right now there is not one place to download a single web extention whats with that???? i wont comment what i think of firefox right now

    1. Will Bamberg wrote on

      > right now there is not one place to download a single web extention whats with that???

      You can install WebExtensions from, just like any other kind of add-on. does not tell you whether an add-on is a WebExtension or some other type of add-on, because that’s seen as an internal thing that users don’t need to know about. But this post: said that there were more than 700, and that was a couple of months back.

      Scott gave you the same answer, essentially, when you asked this question then:

      > bull even a google search has no web extentions

      I’m not sure what you expect to see in a Google search. Searching for “WebExtensions” gives lots of relevant results, including a repository with some examples to look at. But please note the spelling: “WebExtensions”, not “web extentions”.

      1. john wrote on

        thank you for replying so fast your right i did ask that question months ago sorry about that i know you guys all work hard but i dont use firefox anymore as most of the addons dont work now. i use are you watching this and 365 sports notifier tab notifier just to name a few no longer work in the latest firefox editions

    2. gorhill wrote on

      Here is a good place to find WebExtensions-compatible extensions (search for “webextensions”):

      1. gorhill wrote on

        [oops, meant “webextension”, with no “s” at the end]

  2. Kohei Yoshino wrote on

    Posted the Japanese translation:

    1. Jorge Villalobos wrote on


  3. Luís Miguel (Quicksaver) wrote on

    “Don’t dispatch close event, and remove onclose.” – You might want to specify this is relative to workers, there are other “close” things still around I believe, unless I’m wrong?

    1. Jorge Villalobos wrote on

      Yeah, good point. I’ll fix it.

  4. custom.firefox.lady wrote on

    Validation links from the “Add-on compatibility with Firefox 50.*” email show warning “Extension is not marked as compatible with Multi Process” even though “Manage My Submissions” at the Developer Hub shows them all as “Multi Process Status: Compatible”. Bug?

    1. Jorge Villalobos wrote on

      Can you point me to the add-on? It could be a bug.

      1. custom.firefox.lady wrote on

        Multiple add-ons; here are the validation links:

        1. Jorge Villalobos wrote on

          Thanks, I filed this bug report.