It sounds to me that the real headline here is that every copy of firefox out there was timebombed and we only noticed because someone forgot to elongate the fuse.
IMO this seems like just plain bad design. The Firefox addon certificate should never have had an expiry date. If they ever needed to revoke it, they could distribute an updated version of the browser with the previous intermediate explicitly marked as revoked.
xpinstall.signatures.required
Works on Fennec version of Firefox.
Also IceCat version of Firefox wasn't affected AFAIK.
Why? You haven't backed up that statement at all. Especially before they killed XUL it was easy to make a non-doomed app that runs as a browser extension, and it's still plenty possible.
No (non-demo) program should brick itself if it can't connect home.
I don't personally know of any obviously life critical application done this way, mostly because I try to stay as far away from that sort of insanity.
If you don't think it's at least a plausible thing that could eventually happen you haven't been paying attention.
I personally got stuck stranded because of signals stupid built in timebombing when I was relying on a device with no untrusted third party ability to shove silent software updates for communication.