If you don't have a Password Manager already, STOP whatever it is that you're doing and install this add-on! It has a better password generator the Firefox Accounts and also comes with (optional) additional Premium features for $12 like a secure vault (storage space) and you can use it outside of Firefox (multiplatform). On the otherhand, if Firefox is your main browser on all platforms then using the already baked in Firefox Accounts password manager thoretically keeps the attack surface smaller. Moral of the story: pick a password manager and start using it RIGHT NOW! A+
-A very happy Firefox (and Bitwarden) user