Firefox 43 will be released on December 15th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 43 for Developers, so you should also give it a look.

General

• Use channel->ascynOpen2 dom/xbl/nsXBLService.cpp. Comment 64 explains what this has to do with add-ons. Paraphrasing: any content loaded into a webpage needs to have contentaccessible=yes, no matter how it is loaded, which means that add-ons loading XBL into content will probably need to add it.
• Limit @-moz-document to user and UA sheets.

XPCOM

• Remove nsIDOMHTMLCanvasElement::MozFetchAsStream(). This was a non-standard, internal use function, but a few add-ons used it. Give the canvas documentation a look for possible alternatives.
• Use origin attribute in nsIUsageCallback. This makes changes in nsIQuotaManager and nsIUsageCallback.
• A security fix made some changes to the following interfaces: nsIDocShell, nsIDocShellLoadInfo, nsISHEntry. However, these changes should only break compatibility if you are interacting with these components on a binary level.

Signing

• This is the first version of Firefox that will enforce signing. Unsigned add-ons won’t install and will be disabled by default. There’s a preference that turns signing enforcement off (xpinstall.signatures.required in about:config), but the current plan is to drop the preference in Firefox 44.

Please let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 43, 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 42.

https://blog.mozilla.org/addons/2015/11/19/compatibility-for-firefox-43/