Sorry, the Add-ons team is working on a fix for this. It looks like a certificate used to sign many popular extensions expired. (The verification process checks that certificate.)

I don't know how soon re-signed versions of those extensions will become available, or whether another workaround will be discovered. We are all hoping to learn more soon.

The explanation was helpful but I still need my Password manager.

Hi boblesmon, did you install the new Firefox 66.0.4 update, or if you are running the Extended Support Release of Firefox 60, the Firefox 60.6.2esr update?

See: Update Firefox to the latest release

That should re-enable installed add-ons and allow installing new ones. If you run into a glitch, let us know.