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

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/ https://addons.mozilla.org/en-US/firefox/blocked/