yep, I have but with FB4
Allthough IE is my primary browser, I found Firefox works better for debugging from Flex Builder.
You can change which browser FB uses in the preferences.
Window -> Preferences -> General -> Web Browser.
If Firefox is not listed, add it and make sure it is selected.
Wow, I'll have to check this out on Vista. I don't see this on Win2000 with IE. (I know what you're thinking, it's sad yes.)
That would be bad--killing the process in the debugger has always been the best way to stop the app when the debugger is attached.