ALT+F4 might work, but the best way is to do a "ps aux" and get the process number for it and then "kill" it... for example, if your firefox's process number was 4477 you could do a:
if that doesn't work try:
if you don't wanna look for the process number you could try a:
and no, it's not normal for your programs to be crashing/freezing - that's a sign that something is wrong...