UPDATE: Firefox 66.0.4 and 60.6.2esr are now available with the new certificate needed to solve this problem. See: Update Firefox to the latest release.

Sorry, a critical certificate used in verifying add-on signatures expired. Mozilla is distributing a hotfix to re-enable using, installing and updating add-ons. Please check this article to make sure your Firefox is set up to receive the patch(es):

Add-ons disabled or fail to install on Firefox

The hotfix seems to have resolved the problem.. Looking forward to the final fix.

ddenby said

The hot-fix seems to have resolved the problem.. Looking forward to the final fix.

There was a Bug-A-Boo with a Certificate Expiration Date dealing with the Add-ons. There's a new FF Version with the main Fix, 66.0.4.

Mozilla Blog Post Update:
A Firefox release has been pushed — version 66.0.4 on Desktop and Android, and version 60.6.2 for ESR. This release repairs the certificate chain to re-enable web extensions, themes, search engines, and language packs that had been disabled (Bug 1549061).
There are remaining issues that we are actively working to resolve, but we wanted to get this fix out before Monday to lessen the impact of disabled add-ons before the start of the week. More information about the remaining issues can be found by clicking on the links to the release notes above.
(May 5, 16:25 EDT)

~Pj