Clicking Password Button, Saved Password of the website which is opened now show. You can add, copy, edit, and delete password there. In addition, you can check if your email address is safe by Firefox Monitor. If it's a Firefox Relay's address, the Firefox Relay icon shows, clicking open manager of Firefox Relay.
Password auto-saving and auto-insert sometimes doesn't work. And you need enter password to other apps. You can use this in these situation.