Sorry, a critical certificate used in verifying add-on signatures expired, so you need a hotfix to install, enable, or update most add-ons. More info in this article:

Add-ons disabled or fail to install on Firefox

gmajic said

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

I was surprised to find that the only the 32 bit Win 7 OS computer was affected by this bug, the other 2 PC's Win 7 64 bit & the Win 10 64 bit PC's were not affected by this bug.

gmajic said

I was surprised to find that the only the 32 bit Win 7 OS computer was affected by this bug, the other 2 PC's Win 7 64 bit & the Win 10 64 bit PC's were not affected by this bug.

There is a luck factor, because add-ons are not disabled until the first signature verification after midnight May 4th (GMT). That occurs at 24 hour intervals. If you get the new certificate update before that next check, you dodge the bullet.