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

about:config

THEN:

browser.quitShortcut.disabled = true

> Instead of this add-on, visit "about:config" and set "browser.quitShortcut.disabled" to "true" to disable Ctrl+Q/Cmd+Q.
Since many users reported your extension didn't work for them, I went straight for this setting instead. It works perfectly, thank you for this plain instruction, Rob W.

I'd also recommend y'all to read THIS instead: https://superuser.com/a/1352295/599625 -- instructions on how to neutralize Ctrl+Q (and Ctrl+Shift+Q) for numerous versions of Firefox for Linux and Windows.
Note: apparently, this preference also neutralizes Cmd+Q (⌘Q) for Firefox on macOS too, hooray! https://news.ycombinator.com/item?id=29081356#29082023

However, it appears Ctrl+Shift+Q could still appears as a global Quit shortcut in the drop-down menu (I am a Windows user, running Firefox Developer Edition, currently at version 102); I just tested it prior to disabling the preference above: there are no prompts, no warnings, nothing. The whole browser simply closes rather abruptly.

Please try to make your extension disable Ctrl+Shift+Q as well.

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

Thank you for the extension. Works well on macOS Catalina.

Works as described. MacOS Catalina + Firefox 76.

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

Works great on Windows!

Does not seem to be working at all on Ubuntu

Doesn't work on Linux.

Works great, WIndows 10

Does work on my Firefox 73.0.1 on MacOS Catlina.

Does not work on Ubuntu.

Works on Windows 10 with Firefox 66.0.2.

Absolute essential Add-on ! Works perfectly reliable !

Works as expected on macOS, the shortcut no longer quits Firefox.
On GNU/Linux it doens't work as stated in the description, until upstream adds the necessary APIs.

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