Back in August, Mozilla announced plans to adopt a Chrome-like extension framework for Firefox called WebExtensions. In turn, the company plans to deprecate its existing XUL and XPCOM plug-in APIs. These changes are part of a larger project to make Firefox a more modern browser. According to a couple of entries in Mozilla's Bugzilla tracker, some less popular features of Firefox are going to be retired as part of this project, too.
The first feature to hit the trashbin is support for "heavyweight" themes that rely on older add-on technologies. According to the Bugzilla entry for this change, themes that replace chrome packages (not to be confused with the Chrome browser) and "do arbitrary styling" will no longer be supported. Mozilla is mulling the idea of adding features to the lightweight themes supported by WebExtensions, but the entry doesn't have any further details of those plans.
Tab groups will be getting the axe, too. Mozilla says that this feature "has significant effects on more important aspects of Firefox development." The Bugzilla entry says that Mozilla will develop a plan to migrate existing users away from using tab groups.
The response to both proposed changes has been negative, but Mozilla says that the development effort to maintain these features far outweighs the number of Firefox users who actually take advantage of them. While the development team may extend WebExtensions to accommodate themes, it has made no such announcement regarding tab groups. One developer has started work on an extension for groups, though.