Ctl-Alt-F-whatever took a while to clear the screen, but very soon I was able to type "ps aux" to find dozens of Netscapes, dozens of Mozillas, and at least five Galeons rampaging around the CPU. I rebooted after using "killall -12" on all the browsers.
One thing you could do is replace the launcher commands in the gnome panel as follows:
For mozilla,
killall -q mozilla-bin;mozilla
[killall sends SIGTERM by default]
and so forth. Or make wrappers that do this and put them in /usr/local/bin. Or do something that detects that a mozilla is already running and takes no action. But a killall approach should work.