Try using a good browser?
Just kidding... but maybe you have one of those FF plugins causing a problem? I'm guessing other browsers don't do this when you debug?
It's probably just trying to pause the Flash VM when you get an error like it does when you set breakpoints, why this would cause the browser to crash... that's beyond me. Although I guess the VM would have already had to stop... does this also happen when you hit breakpoints in code, or just on an error? Does it happen for only a specific error, or any time you have any kind of run time exception?
Probably the only way you're going to get a really good and definite answer on this is if someone else has already run into and fixed the same exact problem, but I've never heard of it happening before.