as i stated above, firefox extensions are signed using a certificate (for security reasons, as unsigned extensions may potentially be harmful). security certificates in general do not last forever, and typically expire after a set date and time. it appears that they forgot to renew the certificate and the cert just expired, thus making the web browser treat the extensions as invalid.
Although this is true and was actually not stated above, this is not the case as nearly all browser extensions have been effected at the same time by this update. This is most likely a bug with the current Firefox update as stated in the Tweet from ABP which I posted above who's extension is currently being effected by this.
It would be one hell of a coincidence if 90% of all extensions lost their certificate at the same time. But either way, we just have to wait for it to be resolved.