...and reviews by others, and a process by which known bad extensions get taken down.
It's not watertight (as the recent abuses in the chrome web store illustrate), but it's better than nothing.
AFAIK, Chrome extensions don't go through a review process. SmoothGestures was just one extension that got a new developer and then started submitting every visited url to their servers.
Well, there's certainly some central control even if it's not formal review. Of course, review wouldn't catch everything anyway (I think the ability to identify and block bad actors is more important than the a priori review).
It gives a clear path to revocation of bad actors as well as future updates.
It also gives a ton of power to a few large organisations at the expense of small dev shops and advanced users.
Firefox doesn't prevent you from installing code directly. I am stating what I find valuable in their add-on marketplace.
TIL: people too lazy to write three whole words are more interested in sarcasm than saying something sensible.