NOTE: This applies to Firefox 57 on Linux.
- Open a new tab and type
about:configin the address bar and press enter
You will see a warning. Click on the
I accept the risk button.
- Search for the string
showquit. You should see a
browser.showQuitWarning. Double click so its value is toggled to True.
When you press Control Q, there will be a prompt asking you if you actually want to quit Firefox.