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

• Let accel-click and middle-click on the new tab button open a new tab next to the current one. This changes the behavior of the new tab button and removes the BrowserOpenNewTabOrWindow function.
• Code executed in wrong order with setTimeout after alert. This changes the behavior of the onButtonClick function to make it async.
• Drop the prefixed version of visibility API. This removes mozVisibilityState and mozHidden.
• Don’t dispatch close event, and remove onclose. This applies to Workers.
• Prevent installation of an extension when “*” is used as part of strict_min_version in a WebExtension manifest.json). Minimum versions should always be specific versions, rather than a mask.
• Loaded-as-data XML documents should not generate . Some add-ons are relying on this node showing up if an XML document fails to parse.

XPCOM and Modules

• Default to the NullPrincipal rather than SystemPrincipal for Favicons. This changes the functions setAndFetchFaviconForPage and replaceFaviconDataFromDataURL. The principal should generally be passed as an argument.
• Upload setting wrong Content-Type for files if you downloaded from a server that provided the wrong Content-Type. As explained in comment #36, the way add-ons change the MIME type mappings is now different, via a category rather than directly changing mimeTypes.rdf.
• Remove the last references to Actor/FrontClass. While both of these still work for add-ons, they should be considered as deprecated.

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.

https://blog.mozilla.org/addons/2016/10/13/compatibility-for-firefox-51/