Well, uh, one way is to try and install it in firefox.
Assuming you meant without installing it in firefox, I don't quite know. You're going to need to find mozilla's public keys somewhere (maybe just extract them from firefox), unpack the xpi (it's just a zip file with a different extension) and find the signature contained within, and then figure out how to verify it.
Assuming you meant without installing it in firefox, I don't quite know. You're going to need to find mozilla's public keys somewhere (maybe just extract them from firefox), unpack the xpi (it's just a zip file with a different extension) and find the signature contained within, and then figure out how to verify it.