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.

Thanks for the quick reply. I thought it was so strange so many simple add-ons such as themes became invalid. I'll keep checking updates. Thanks again.

There is a hotfix released for the issue about an expired intermediate certificate that caused extensions to get disabled that is currently applied via a shield study and you only get this hotfix when studies are enabled.

See also:

• https://support.mozilla.org/en-US/kb/add-ons-disabled-or-fail-to-install-firefox
• https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/

Thanks Cor-el! That worked for now!