Ctrl + Q inderectly conflicts with Ctrl + W which is used to close tabs, in that sense it’s easy to accidentally hit the former and kill your entire browsing session, all windows included. Navigating only with the mouse massively slows down my workflow.
There are at least the browser.sessionstore.warnOnQuit and browser.warnOnQuit about:config preferences, but both had no effect when set to true on my system.
Not an answer to your question but I use ctrl-F4 to close tabs.
Not an answer to your question but as workaround you could tell FF to remember all open tabs. So if you accidentally close it, it will reopen all tabs again upon start.
Again, not a direct answer, but Firefox can be set to warn (and by default does) about open tabs – which prevents the Ctrl + Q hotkey from closing the application on its own.
The related about:config preferences (to true) are:
browser.tabs.warnOnClose
browser.tabs.warnOnCloseOtherTabs
services.sync.prefs.sync.browser.tabs.warnOnClose