Firefox 21 will be released on May 14th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 21 for Developers, so you should read that too.

General

Places

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 21, I’d like to know.

The automatic compatibility validation and upgrade for add-ons on AMO will happen very soon, so check your email if you have an add-on listed on AMO.