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One thing that's IMO wrong with the way firefox checks addon signatures is that it checks signatures on already-installed addons periodically.

Why?
What's the point?

If a malicious addon was running 5 minutes ago, what do you gain by disabling it now?

And if it wasn't malicious 5 minutes ago, it's probably isn't malicious now. The code is still the same.

Install time checing - sure.
Checking on startup - fine.
But checking after you've already executed it?