Firefox 49 will be released on September 13th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 49 for Developers, so you should also give it a look.

General

• Cleanup doorhanger notification anchor ID & class duplication. This could affect the styling of add-ons that use popup notifications.
• Make a window opened through window.open() to be scrollable by default.
• Remove non-standard flag argument from String.prototype.{search,match,replace}.
• Remove HTML Microdata API.

XPCOM and Modules

• Limit the size of pref values sent to the child process. As a reminder, you shouldn’t use the preferences service to store large amounts of data. It should only be used for small values (under 4Kb). This bug enforces it for inter-process communication.
• Need to shim protocol.js move for add-ons. The module should now be accessed using
  require("devtools/shared/protocol"). For now, there’s a shim that permits the old location to be used.
• Add deprecation warning within nsFaviconService.cpp. If you manipulate favicons, you can find the changes described in this bug comment. After this change you’ll only see a warning if you don’t pass the right principal, but eventually it won’t work as you expect.

Let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 49, 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 48.

https://blog.mozilla.org/addons/2016/06/30/compatibility-for-firefox-49/