You know about Components.Exception, right? Might be useful for you.
http://developer.mozilla.org/en/docs/nsIXPCException

(From Alex: Actually, no, I'd not seen that in any detail before. I can't find any good documentation on it through a cursory search, and no one seems to be using it... help!)

The documentation I linked to is all there is - and I patched about half that together.

It is what is recommended to be used when throwing exceptions from Javascript Components. I can't recall if that was Mook or someone else telling me that, but I found out while working on that mozSubversionClient component.

(From Alex: Hm, okay. Except it literally isn't being used anywhere in the Mozilla code, as far as I can tell. So there could be any number of bugs there.

I'll think about it.)