Use the "passwords" feature to check if one of yours is compromised. If it shows up, never ever reuse those credentials. They'll be baked into thousands of botnets etc. and be forevermore part of automated break-in attempts until one randomly succeeds.
There's an add-on for the browser for both, but on Mac, the desktop app is what integrates with the system wide password manager. I don't know if desktop Firefox is integrated into that, so you may need both the add-on and desktop app to get the same systemwide functionality.
On Windows it's worth having both the browser add-on and desktop app installed as well, since the browser add-on only works in browser but the desktop app, while somewhat hit or miss whether or not it works with any specific application, is supposed to provide autofill/generation capabilities anywhere you have username/password field.