Firefox enables user to install add-ons so as to increase the utility. Firefox ensures that the add-ons(which are basically third party code) are reviewed and signed (with a Mozilla certificate). These signed add-ons are available on addons.mozilla.org and also can be hosted on developers website. Now once you install the add-on in the Firefox, it looks for the certificate and allows only those add-ons which are signed by known Mozilla certificate. Rest every add-on is shown as corrupt.
Unfortunately the certificate is expired for now and hence all add-ons show as corrupt, Firefox add-ons team is working on it as of now and soon we will have a fix. Bugzilla entry
Follow this twitter thread for the fix progress (mozamo)[https://twitter.com/mozamo/status/1124484255159971840]
As a really quick workaround you can try ideas on this reddit thread but obviously these should be temporary!
Update
Latest comments (3)
This fixed it for me discourse.mozilla.org/t/certificat...
other browser vendors don't sign extensions, right?
Yes atleast to my knowledge Chrome and Opera does.