I'm unable to download or install any extensions within Firefox.
When attempting to install any extension through Firefox addon's I receive a "Download failed. Please check your connection" message.
When attempting to install directly from an extensions website I receive a corrupt addon pop up box.
I've tried uninstalling my AV (Avast), changed the "xpinstall.signatures.required" from true to false.
Extensions install successfully on other browsers (Edge).
Firefox 66.0.3 64-bit Windows 10 Home Version 1809
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.
This isn't a problem on your end. This is happening to quite a few people, probably everyone I'd guess. This is only a temporary problem and they are working on fixing it. You are not alone. I'm just glad I found this forum. I was able to get answers, and now I can help other people.