You can also tell Firefox to ignore it completely:

    browser.quitShortcut.disabled

As well as to warn:

    browser.warnOnQuit
    browser.warnOnQuitShortcut

Well, apparently I once was aware of these because I have it set in my custom user.js. But I guess ctrl-q will always be lock screen for me, old habits die hard.

and testing it now, it asks me by default, and I never changed that setting