Earlier quoted context omitted.
But that represents how people consider trust when choosing addons. It's trusting the code and company at the time of install, not at an arbitrary later time. Sure, if the cert expires and there's an update then the user wants to know.
The main trust check is at installation time, but it's possible for problems to be discovered later, and Mozilla needs to be able to do something about it. Certificate non-renewal is the only robust avenue of revocation.
It's exceedingly poor ethics.