Firefox 41 will be released on September 22nd. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 41 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. Firefox 40 only warned about it. 41 will disable unsigned extensions by default. All AMO add-ons have already been signed and we’re in the process of reviewing non-AMO add-ons.

General

• Remove the browser.newtab.url preference.
• Date.prototype should be a non-Date Object.
• Remove SpiderMonkey support for let expressions. If you’re unclear about let expressions, you’re probably unaffected. Nevertheless, here are the docs.
• Method definitions require curly brackets.

XPCOM

• Turn NewChannel deprecation warnings into assertions. We have warned about this before, and you can find the documentation of the new interface here.
• Get rid of nsIDOMFile.
• Consolidate nsIDebug and nsIDebug2 interfaces.
• Convert nsIHttpChannel to builtinclass.

New

• Add webRequest-like API to Firefox.
• Create “speed dial” dynamic home panel layout. This new layout is available for add-ons in Firefox for Android.

Please let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 41, 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 40.

https://blog.mozilla.org/addons/2015/08/14/compatibility-for-firefox-41/