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

• Add ability to modify a list of built in themes for the Lightweight Theme Manager. Add-ons that use the lightweightThemes.usedThemes and lightweightThemes.isThemeSelected preferences are affected, as explained on comment #39.
• Add console warnings for non-standard flag argument of String.prototype.{search,match,replace}. Note that this is just a deprecation warning, but the last “flag” argument will be removed eventually.
• Remove the -remote option. This removes the -remote command line switch, not to be confused with the more often used -no-remote.
• Warn when __noSuchMethod__ is used. Whilte it still works, it’s on its way out, so you should look for alternatives as suggested in the bug comments.

XPCOM

• Remove XMLHttpRequest sendAsBinary.
• Change nsIDocumentLoaderFactory and nsIURIContentListener to take MIME types as an XPCOM string.
• Make nsIClassInfo::getHelperForLanguage specific to JavaScript. This removes the getHelperForLanguage function.

New!

• Add a simple mechanism for content pages to communicate with chrome. There’s a good explanation of this in Mossop’s blog and on MDN.

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.