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.
The real answer to potentially destructive moves is not to ask the user twice, but to let the user easily undo.
I've fat fingered Cmd-W and Cmd-Q too many times, and while it's easy to restore, it takes a couple minutes, a lot of bandwidth, and spins the CPU to 100% for a while. Which really sucks when you're on battery.
You can disable it though.
On Windows it silently keeps running in the background most of the time if you have apps or notifications running.
I use ctrl-q as my shortcut key in tmux. I can't count how many times I accidentally sent that into the wrong window :) I consider showQuitWarning in Firefox a necessity.
IMO, no application ever should quit from one key combination without confirmation.