Sorry, the Add-ons team is working on a fix for this. It is not specific to particular themes and extensions, it looks like a certificate used to sign many popular add-ons expired. The verification process checks that certificate, and anything that doesn't pass is blocked.

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

Mozilla has rolled-out a fix for this. The fix will be automatically applied in the background within the next few hours, you don’t need to take active steps.

In order to be able to provide this fix on short notice, they are using the Studies system. You can check if you have studies enabled

1. Go to [=] > Options > Privacy & Security.
2. Make sure Allow Firefox to install and run studies is check marked.

I you had it disabled, you can disable studies again after your add-ons have been re-enabled.

They are working on a general fix that doesn't need to rely on this and will keep you updated.