This happened to me as of two days ago, and when I did a search I found out that many many many people are frustrated and complaining about this. I use several extensions to do my work, and understand the challenges.
“Certificate issue causing add-ons to be disabled or fail to install”
[from this Mozilla blog page] “Late on Friday May 3rd, we became aware of an issue with Firefox that prevented existing and new add-ons from running or being installed. We are very sorry for the inconvenience caused to people who use Firefox.
“Our team has identified and rolled-out a fix for all Firefox Desktop users on Release, Beta and Nightly. The fix will be automatically applied in the background within the next few hours. No active steps need to be taken to make add-ons work again. In particular, please do not delete and/or re-install any add-ons as an attempt to fix the issue. Deleting an add-on removes any data associated with it, where disabling and re-enabling does not.“