First thing I do on every new Mac - remap the Cmd+Q command that closes all the things (one fat finger away from Cmd+W which we all use daily) to something harmless, like invert colors. lifesaver.
I just check with Firefox which he was complaining about, and while it doesn't do this, it at least by default prompts you to confirm you want to close multiple tabs which should hopefully prevent you from quitting out by accident.