I worked around the accidental-Command-Q problem in Firefox by setting up a custom keyboard shortcut using the paid app Keyboard Maestro (https://www.keyboardmaestro.com/). I have a macro “Confirm Command-Q to Quit” that intercepts the ⌘Q keystroke, only in Firefox, and instead shows a floating dialog titled “Really quit Firefox?”. The Cancel button in the dialog stops the macro, and the Quit button continues the macro to the next step, which is a Quit Firefox instruction.

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.