pgrep firefox listed two processes.
killing these processes by kill
killall -9 firefox-bin
did the trick.
for the record:
This is an oldie but a goodie:
1. When a program gets hung, hit Alt + F2 to bring up the Run dialog.
2. Type in xkill and hit enter.
3. Your mouse cursor will change into a cross or skull and bones.
4. If the unresponsive application is not in focus, use Alt-Tab to switch to it. Left click anywhere on the application window to kill it.