> 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.

Doesn't seem to work any more with new Firefox 57 :-(

Sorry, I don't know how to reply here, so I hope you see my edit: I am using Ubuntu 16.04. The plugin loads, but Ctrl+Q still closes the browser. Let me know if you need more info and thanks for any help!