It’s in a lot of program, not just ff. You can also see some letters are underlined in that menu, if you press that letter after alt, it would invoke that command or open that drop down without using the mouse. This is a convention at least from DOS, but I suspect it may be even older.
So actually alt doesn’t unhide the menu, it waits for a letter input to what command you want to start. It just happened that this old type of menu is hidden by default in a lot of programs and alt could be reused for this as well.
There is a community for these: !lemmyapps@lemmy.world