Firefox 50 will be released on November 8th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 50 for Developers, so you should also give it a look.

General

• Remove box-sizing: padding-box.
• Forbid CPOW usage if add-on declares it is multiprocessCompatible. The multiprocessCompatible flag in the manifest disables all cross-process shims, ensuring the add-on works completely in e10s mode. This bug closes a loophole in this restriction. If you’re unfamiliar with e10s compatibility, please give this a read.
• Remove draggesture and dragdrop events. Their standard equivalents should be used: dragstart and drop.

XPCOM and Modules

• nsIClientAuthDialogs::ChooseCertificate should pass an nsIArray of nsIX509Certs, not strings. This change removes the CERT_USAGE_* constants in nsIX509Cert, used by some add-ons.
• Remove nsIX509Cert.getUsagesArray, requestUsagesArrayAsync, and getUsagesString.

New

• Provide API for add-ons to delay restartless updates. Delaying updates can be useful if an add-on is in the middle of running a slow process that needs to run completely.

Let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 50, 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 49.

https://blog.mozilla.org/addons/2016/09/01/compatibility-for-firefox-50/