Apparently, at the moment, extensions installed in Firefox for all users have been disabled and users can not also install new extensions from AMO, this has happenned due to certificate issue which Mozilla acknowledged and is working to fix.
Just check your Firefox browser now, it doesn’t matter it is stable or Nightly, extensions may have been already disabled by showing notice that “one or more instaled add-ons cannot to be verified and have been disabled”.
And if you now visit Add-ons.Mozilla.org and try to install an add-on, you’re out of luck, the add-on installation will be prevented by saying “the add-on downloaded from this site could not be installed because it appears to be corrupt”.
Update May o4, 2019: Mozilla rolls out Hotfix for Firefox add-ons issue, here is what you need to do for it to work
End.
Huge mistake! Mozilla guys allowed an intermediate signing certificate for Firefox to expire, now all add-ons have unverfied stauts, since Firefox doesnt allow unsigned extensions, new ones won’t be installed and already installed extensions will be disabeld. At the moment, you can re- enable your extensions in Nightly, for that,
1. Visit about:config
2. Find xpinstall. preference and toggle its vlaue to false
3. Restart FIrefox Nightly and visit add-ons manager to notice extensions enabled.
Mozilla provided this information here for “Certificate issue causing add-ons to be to disabled or fail to install: “At about 6:10 PST we received a report that a certificate issue for Firefox is causing add-ons to stop working and add-ons install to fail.Our team is actively working on a fix. We will update as soon as we have more information”.
You can follow this bug for udpates.
Related articles:
Mozilla introduces extension Signing for Firefox
Firefox 40: Add-ons Manager shows Warnings for Unsigned Add-ons
How to disable Extension Signing in Firefox 40
Mozilla delays the Removal of Add-on Signing disable preference to Firefox 46