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.

General

XPCOM and Modules

New!

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.