I'm surprised that functionality isn't possible to disable, but tbh I really like it. Too many times a finger slip turns Cmd+W into Cmd+Q and suddenly I'm lost a whole pile of tab state, possibly even half-submitted forms. Yuck.
And it's so rare that I shut down the whole browser anyway, so it makes sense to me to make it hard to do accidentally.
Another possible workaround is to go to System Preferences > Keyboard > Shortcuts > App Shortcuts and create a new shortcut. You can specify that in the app Firefox, the menu item “Quit Firefox” should have the shortcut ⌥⌘Q. Then a normal ⌘Q should do nothing.