Confirmed, sorry for that, I hope that you did not accidentally lose your tabs. I have edited the add-on description to state that it does not work on Linux for now until https://bugzilla.mozilla.org/show_bug.cgi?id=1325692 is fixed.

For now, you can use https://addons.mozilla.org/en-US/firefox/addon/disable-ctrl-q-shortcut/. That add-on will cease to work by the end of this year though. Hopefully the linked bugzilla issue will be fixed by then, so you can install this add-on again to get it to work (or keep the add-on installed; although it does not work right now, it does no harm either).

If you want to manually prevent the Ctrl-Q shortcut from working, open the global JavaScript console (Ctrl-Shift-J) and run the following code. This needs to be done for every new window.
document.getElementById('key_quitApplication').remove()