Mozilla announced major upcoming changes to Firefox add-ons on the official Add-ons Blog today. These changes affect add-on developers and Firefox users alike, and will have a major effect on add-on compatibility and permissions.
The four major changes that Mozilla mentions explicitly in the announcement are add-on validation and signing, the multi-process architecture Electrolysis, the implementation of a new extension API WebExtensions, and the deprecation of XPCOM and XUL based add-ons.