What to do: In about:config , toggle browser.warnOnQuit from false to true
Why:
Default behavior when hitting <cmd>q in Firefox on MacOS X is to unconditionally quit Firefox. Note: The Apple <cmd> key has the
same location on a German keyboard as <AltGr> on a industry standard PC, and <AltGr>q creates a @ symbol by default on such a
PC (Linux and Windows).
So it can happen that you want to enter a @ symbol as part of an email, and by accident, when pressing the industry standard keys
on a Apple computer, your browser quits instantanteously. As a consequence, all your data in all your browser tabs will be lost.
Activating the Warn you when quitting the browser setting in Preferences -> General -> Startup does not cause Firefox to create
a warning popup after hitting <cmd>q.
Affected version(s): 78.2.0esr (64-bit) and many previous versions as well
Solution in: https://bugzilla.mozilla.org/show_bug.cgi?id=1438499#c5
Sample search engine search texts for displaying related bugs: firefox mac os always quits without asking with cmd-q Warn when quitting the browser setting has no effect on Firefox on MacOSX