I clicked "Stop Script" and it locked up again. Eventually it came back up with the same prompt. Fortunately I had another FF window open, which I used to open Firebug and disable the Net, Console and Script tabs. The browser then came back to me. Oddly, Firebug isn't enabled for Youtube, so I'm not entirely sure why it affected this tab.
Anyway, my point is simply that something, whether it be Firefox or Firebug, decided that it was going to disregard the fact that I'd told it to stop running the offending script, but it continued. Annoying!