I'd expect Mozilla to do that soon as the issue is widespread and affecting lots of Firefox users. The organization needs to renew the certificate or create a new one to resolve the issue.

The issue can only be resolved on Mozilla's end. You could change the system date to the previous day to resolve it temporarily as well, but that can lead to other issues. You need to set the preference to false on about:config to disable signing.

Nightly, Dev and Android users may be able to disable signing of extensions some users reported that this resolved the issue temporarily on their end. Please check your connection' errors when they attempt to download any extension from the official repository.

Related issues have been reported: some users cannot install extensions from Mozilla's official Add-ons repository. Firefox will block the installation of extensions with invalid certificates (or none), and that is causing the issue on user systems. All Firefox extensions need to be signed since Firefox 48 and Firefox ESR 52.

Mozilla firefox extensions location