Nope, if the SSL industry continues to insist on increasingly short cert lifetimes then I want Firefox to give no quarter when a cert expires.
Play by their rules and fall by their rules too.
The former is most likely an administrative error (ie: someone forgot to renew, or the auto-renew is failing). The latter is more likely to be an MTM attack.
I'm not sure how you would use an expired cert as an attack vector. By loading in an old cert into an expired domain so you could spoof older content?
Expiry is a pretty fundamental part of the security model of certificates.
Also of course domains changing owners, but again... I don't think we have good monitoring for that during the current long lifetime, so maybe a grace period where a warning is shown but it's easier to click through would be a good idea. Perhaps combined with a requirement to keep revocation information (and keep revoking expired certificates) X days past expiry.