Firefox 39 will be released on June 30th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 39 for Developers, so you should also give it a look.

General

XPCOM

New!

Please let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 39, I’d like to know.

The automatic compatibility validation and upgrade for add-ons on AMO will happen in the coming weeks, so keep an eye on your email if you have an add-on listed on our site with its compatibility set to Firefox 38.