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.