Instead of this add-on, visit about:config and set browser.quitShortcut.disabled to true to disable Ctrl-Q/Cmd-Q. (Firefox 87+ only, since bug 52821 got fixed). I have closed Firefox too often by accident while pressing Ctrl-Q (Linux/Windows/Mac) or Command-Q (Mac). This is especially inconvenient in private browsing mode, because such tabs are not restored by session restore when you restart Firefox. This add-on replaces the Ctrl-Q and Command-Q shortcuts, so that upon pressing these shortcuts, Firefox does not quit any more. This add-on does not work as expected in Linux, until bug 1325692 is fixed. It works on macOS and Windows. Instead of this add-on, visit about:config and set browser.quitShortcut.disabled to true to disable Ctrl-Q/Cmd-Q.