Firefox users started to notice that installed browser extensions were all disabled in the web browser suddenly. Extensions would display “could not be verified for use in Firefox and has been disabled” messages in the add-ons manager of the browser. Firefox would display “One or more installed add-ons cannot be verified and have been disabled” at the top as a notification next to that.
Turns out, the issue is caused by a bug. If you read carefully, you notice that verification is the issue. A new thread on Bugzilla suggests that this has something to do with extension signing.
Firefox marked addons due signing as unsupported, but doesn’t allow re-downloads from AMO → All extensions disabled due to expiration of intermediate signing cert.
All Firefox extensions need to be signed since Firefox 48 and Firefox ESR 52. Firefox will block the installation of extensions with invalid certificates (or none), and that is causing the issue on user systems.
Related issues have been reported: some users cannot install extensions from Mozilla’s official Add-ons repository. Users get “Download failed. Please check your connection” errors when they attempt to download any extension from the official repository.