The add-on just overrides Ctrl-Q and ignores the keystroke. I don't know if there is a way to exxplicitly quit Firefox (I can close all open windows, but I don't know for sure whether the windows would be restored when Firefox starts again).
I believe that quitting is such a rare operation that it should not be possible to do it by accident. Two Cmd-Q presses can still be done by accident.
If you want to explicitly quit Firefox by keyboard, you can use the following shortcut:
- Linux (and Windows too, I guess): Alt + F, Q
- macOS: Fn + Ctrl + F2, Arrow right, Arrow Down, Q, Enter (Ctrl instead of Fn + Ctrl if you are not using a Mac keyboard; to enable Ctrl+F2 the menubar shortcut should be enabled, see https://superuser.com/questions/303525/what-is-the-shortcut-to-access-the-menubar-in-mac-os-x).
(by following these shortcuts, you are basically opening the menu and activating the Quit menu item)