How much history is stored on that computer?
If there is a lot of history or if you computer is slow, Firefox may hang while it works. In this case, you should allow Firefox to remain frozen for a bit.
If Firefox hangs for a long amount of time, please stop the Firefox program.

Please report back soon.

In most cases, this is normal.

I have this problem, and it is most definitely NOT normal. It's fairly new, don't know which version it began with, but it's a real issue. I have OS X 10.6.8 on an iMac. I have FF 26, this has been happening for several versions now. I almost never can clear history, even just to empty the cache, without FF hanging. It happens whether I have a lot in history or almost nothing. Sometimes I get the non-functioning script message, sometimes not, but clicking Cancel Script or Continue makes no difference anyway. And when FF hangs, I have to Forcequit FF - which always shows up as not responding. And get this: when I Forcequit, it puts my computer to sleep! It all goes black and shuts down, but not off. Can't wake it with the keyboard or mouse - I have to press the power button and release quickly. This is bizarre, and no, I don't have any bugs or viruses. Mozilla introduced this glitch a while back and it's about time they dumped it.