No need for this! You can do on Firefox 87+ versions:

about:config

THEN:

browser.quitShortcut.disabled = true

Does not seem to work on Windows 10, Firefox 85.0.2. I installed the add-on and tried Ctrl+Shift+Q...and my browser closed. I thought, okay, maybe it needs a restart to work. So I opened it back up and tried it again...same thing. I was on a regular web page when I tried it, not a special browser page (which I think add-ons don't normally work on).

If I did something wrong or if this is fixed, let me know, and I'll change my rating.

Does not work on Pop!_OS 20.04 LTS

Does not work on Linux.

Doesn't work on linux. you say to read the description if you're on linux but there's nothing there

Does not seem to be working at all on Ubuntu

Doesn't work on Linux.

Does not work on Ubuntu.

Doesn't work. Ctrl-q still quits Firefox 62.0.2.

it doesn't work

Doesn't work on Ubuntu!

doesnt work

STILL does not work on Linux.

It doesn't work. At all.

Does not work (Firefox 61.0.1 on Linux).

This addon does not work. It allows ctrl+q and cmd+q to both close Firefox.

I used to take advantage, before FF Quantum, of the great "Warn before quit" plugin, that was much more comfortable: you were simply prompted to press CMD-Q again if you were sure of quitting.
Here it's to basic to just disable CMD-Q! At least, there should be the option to quit otherwise, for instance by Alt-Q or whatever else.

You had one job
(Ubuntu 18.04, FF60)

Does not work on Ubuntu w/ FF 60.0

Essential addon for FF, unfortunately not working in Ubuntu

Still not working on linux based systems.

Doesn't work