User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:66.0) Gecko/20100101 Firefox/66.0

Steps to reproduce:

I accidentally pressed Ctrl+Q instead of Ctrl+W.

Actual results:

Firefox window with all its tabs disappeared and application terminated.

Expected results:

Nothing.

Serious user experience defect!

Just search for "Ctrl Q Firefox" on the web and you should be convinced by the number of hits.

Ctrl+Q should do nothing by default, which is how Chromium behaves, hence I am switching to it.

You should fix this, unless you want to loose more users to Chromium. It should not be difficult to change the keybinding default for someone familiar with the codebase.