Firefox 51 will be released on January 24th. Note that the scheduled release on December 13th is a point release, not a major release, hence the much longer cycle. Here’s the list of changes that went into this version that can affect add-on compatibility. There is more information available in Firefox 51 for Developers, so you should also give it a look.
- Let accel-click and middle-click on the new tab button open a new tab next to the current one. This changes the behavior of the new tab button and removes the
- Code executed in wrong order with
setTimeoutafter alert. This changes the behavior of the
onButtonClickfunction to make it async.
- Drop the prefixed version of visibility API. This removes
- Don’t dispatch close event, and remove onclose. This applies to Workers.
- Prevent installation of an extension when “*” is used as part of
strict_min_versionin a WebExtension
manifest.json). Minimum versions should always be specific versions, rather than a mask.
- Loaded-as-data XML documents should not generate
<parsererror>. Some add-ons are relying on this node showing up if an XML document fails to parse.
XPCOM and Modules
Default to the NullPrincipal rather than SystemPrincipal for Favicons. This changes the functions
replaceFaviconDataFromDataURL. The principal should generally be passed as an argument.
- Upload setting wrong Content-Type for files if you downloaded from a server that provided the wrong Content-Type. As explained in comment #36, the way add-ons change the MIME type mappings is now different, via a category rather than directly changing
- Remove the last references to
FrontClass. While both of these still work for add-ons, they should be considered as deprecated.
- Embedded WebExtensions. You can now embed a WebExtension into any restartless add-on, allowing you to gradually migrate your code to the new platform, or transition any data you store to a format that works with WebExtensions.
- WebExtension Experiments. This is a mechanism that allows us (and you!) to prototype new WebExtensions APIs.
Let me know in the comments if there’s anything missing or incorrect on these lists. If your add-on breaks on Firefox 51, 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 50.