I, too, would like to see this implemented. On Firefox Android (stable and nightly) the UX to copy a stored password is pretty cumbersome at the moment as you have to go to ... -> Settings -> Logins and passwords -> Saved logins -> Search (or scroll through list) -> Tap to open -> Copy
Also the password manager doesn't remember the last search so if you select the wrong site or credentials and go back, you have to search again.