Earlier quoted context omitted.
Cert check doesn't need to connect to any url to fail. It's just the cert's only expired 20 hours ago, so this issue is currently only affecting approximately 83% of users who happen to be checking at a time of day that has already passed, over the next 4 hours that will go up to 100% (ignoring users who receive a fix before it breaks).
ah, good point. i didn't think through beforehand how this should have worked.
turns out i had `xpinstall.signatures.required` set to `false`, which i'd done to get an older extension working in the past (and subsequently forgot to set it back to `true` later). so i don't think my install of firefox has been checking addon signatures for a while now, which is why my addons remained functional (with other security implications of course).
i didn't immediately find where the cert was stored (to check it's expiration date) however.