Mozilla’s bug tracker, Bugzilla first suggested the sudden failure in the code signing certificate built into the Firefox browser, which lapsed just after midnight on May 4, causing the re-installation of extensions to fail — with users getting different error messages.
While the company neglected to renew the certificate, Firefox browser treated the extensions as no longer trusted and potentially a malicious program, so disabled any already installed add-on on the browser for that same reason.
The resultant effect was that the extension is listed as a “legacy” extension, with warning that it “could not be verified for use in Firefox and has been disabled” for security reasons.
Mozilla, however promptly crafted a temporary fix for the desktop versions of Firefox and pushed out the patch to the browser using the Studies system. It uses Studies to push out test code to a subsection of the Firefox user base, and also to collect data on users' reactions to its sponsored content.
The issue has now been fixed in the standard desktop version of the browser, though some versions (like Firefox for Android) will still require a separate update. And the patch will be automatically applied in the background with no active steps needed to be taken from the users to make add-ons work again. Albeit, it is recommended that users should not delete and/or re-install any add-ons in attempt to fix the issue.
It is worth to mention that Firefox gained a full percentage point this past April, ending the month with 10.2% user share, the highest since March 2018 and the first above 10 points since mid last year. This is nearly two years since Firefox has had a sustained period of rise (about five months, from March to July 2017).