Firefox 40 will be released on August 11th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 40 for Developers, so you should also give it a look.

Extension signing

This is the first version of Firefox that will enforce our new signing requirements. All AMO add-ons have already been signed and we’re in the process of reviewing non-AMO add-ons. In this version, there’s a preference to disable the signing requirement (xpinstall.signatures.required), which will be removed in 41.

General

XPCOM

Themes

Please let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 40, 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 39.