Mozilla claims that they review all the code, but in practice they don't. They have an automated tool that looks for certain things. If nothing is found, this is the end of the code review. If things are flagged, then a human will look at the code a little.

If you have an established extension and push an update, odds are there will be no human review of the code changes. That is how most malicious extensions happen.

Sure, Mozilla historically had less malicious extensions than Chrome. But that's for the same reason that Linux has less viruses than Windows: hackers will target the 90% of users and not waste time on the rest.

I say all this as a staunch Firefox user and maintainer of a handful of extensions.