Firefox 56 will be released on September 26th. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 56 for Developers, so you should also give it a look. Also, if you haven’t yet, please read our roadmap to Firefox 57.
- Convert remaining XUL DOM classes to WebIDL. This could affect add-ons that do things like
t instanceof aWindow.XULControllers, which should be rare.
encoding_rs. This could cause problems if you’re using converter services to convert to UTF-16. Comment 121 explains a little more.
nsStandardURL‘s internal hostname representation to punycode. This could cause some breakage when manipulating hostnames that use non-ASCII characters.
Let me know in the comments if there’s anything missing or incorrect on these lists. We’d like to know if your add-on breaks on Firefox 56.
The automatic compatibility validation and upgrade for add-ons on AMO will run 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 55.
Firefox 56 will be the last version of Firefox to support legacy add-ons. It’s the last release cycle you’ll have to port your add-ons to WebExtensions. Many planned APIs won’t make the cut for 57, so make sure that you plan your development timeline accordingly.
This is also the last compatibility overview I’ll write. I started writing these 7 years ago, the first one covering Firefox 4. Looking ahead, backwards-incompatible changes in WebExtensions APIs should be rare. When and if they occur, we’ll post one-offs about them, so please keep following this blog for updates.