Since it's about addons (and not https connections) the same approach as with code-signing certificates could be used. For code signing it only matters that the certificate was valid at the time of signing, not when the signed resource is used.

E.g. when the certificate expires, any resource signed with that certificate while it still was valid continues to be usable, you just can't sign any new releases with the expired certificate.

I'm asking why my computer cares what time it is before it executes a piece of code. I put it there. Mozilla gave me permission at the time. I shouldn't need it again later. Code signing is DRM.