At least Firefox extensions on Mozilla's add-ons site gets more thoroughly reviewed on every update.
The add-ons installed from outside of the add-ons site can be very dangerous, but Mozilla tries to block these too: List of blocked add-ons with reasons: https://addons.mozilla.org/en-US/firefox/blocked/
So i tried it, and sure - i was even able to replace password logins in the DOM with fake ones.
Firefox extensions does the same thing really, so now i only use a few "safe" extensions.
I'm surprised that this hasen't gotten more attention.