The pattern is essentially xhr+eval... really it's the eval that's the problem... Firefox in particular has no way to track code from an eval back to its original source and instead points at the eval call site, plus whatever line offset there is in the eval buffer. Firebug has implemented a clever scheme to workaround this problem, and added an optional hint which loaders like Dojo can use to embed the original file path in a comment. Webkit now supports this scheme also. It's not perfect, but debugger; and other breakpoints ought to bring you into the correct buffer.
I'm not sure why none of this would be working for you. Which version of Firebug are you using?
