if u wanna run it again without restarting ur pc, then press Ctrl+Alt+Del and the task manager will appear....
click on the tab "processes"
find 'firefox.exe' in the list below....click on it and then click on end process....it will be closed...now u can again start it...
when u now open firefox, it will ask u if u want to restore previous session(dats because it encountered some problem) and u can easily restore all ur work...