If you haven’t yet, please read our roadmap to Firefox 57.

Firefox 54 will be released on June 13th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 54 for Developers, so you should also give it a look.

General

Remove -moz-appearance . This doesn’t apply to CSS sheets loaded using a chrome:// URL, but it does affect inline CSS styles in XUL and JavaScript code.

XPCOM and Modules

WebExtensions

Update: sessions.restore returns an array instead of an object. The return value is now an object, matching what the equivalent Chrome API does.

returns an array instead of an object. The return value is now an object, matching what the equivalent Chrome API does. Implement runtime.onMessageExternal / onConnectExternal . Quoting comment 34, this changes the meaning of the browser.runtime.id property, which may cause issues for extensions which expect that to be the same as the ID used in their URLs.

/ . Quoting comment 34, this changes the meaning of the property, which may cause issues for extensions which expect that to be the same as the ID used in their URLs. webRequest doesn’t check host permissions. You’ll need to add the necessary permissions for webRequest to continue working. At the moment this is incompatible with the Chrome API.

Let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 54, I’d like to know.

The automatic compatibility validation and upgrade for add-ons on AMO will happen in a few weeks, so keep an eye on your email if you have an add-on listed on our site with its compatibility set to Firefox 53.