Big surprise for users of the Firefox browser who could see that extensions (or add-ons) were no longer usable because they were considered obsolete. There is a problem with the certificate.
This certificate issue that became invalid affected the execution of existing or new extensions, as well as the installation of extensions. No need to attempt any hack, a patch is being deployed in the background.
We rolled out a hotfix that re-enables affected add-ons. The fix will be automatically applied in the background within the next few hours. For more details, please check out the update at https://t.co/G5ypnckjdM
– Mozilla Add-ons (@mozamo) May 4, 2019
Some affected Firefox users are already starting to find the use of their extensions.
To speed up the distribution of the patch, Mozilla uses the Studies system, which is activated by default. If necessary, go to the Firefox settings for privacy and security, and authorize Firefox to install and launch studies.
Once the fix has been obtained (which may nevertheless take time), the parameter (for studies) can be deactivated if necessary. Mozilla is working on a global fix that will not require relying on the Studies system.
Note that to find out if the intermediate certificate (the fix) has been obtained, simply enter "about: studies" in the address bar. It is identified as hotfix-update-xpi-signing-intermediate-bug-1548973.