When the script causes an exception, Firebug shows an error message in the console and stops. However, there is no stack trace and nothing in the watch panel. The exception happens in a function that is called from many other functions.
I've tried setting Break On All Errors, it makes no difference. I've set the Show Stack Trace With Errors option, still no difference. I could use the error message in the console to set a breakpoint on the line that causes the exception, only this stops every time the function is called. Which means I can't run the script long enough to generate the exception.
This leads to the second problem. Once I've set a breakpoint from the console, execution always stops at that line but the breakpoint dosen't show in the script view. I can't find a way to remove it.