Patrick Walton has written a “JS Deminifier“, which undoes some of the work of minification to make your production JS easier to read.
What if, though, the minifier were to dump out its internal state, in the form of mappings from long names to short names and whatever other info was necessary, in a way a deminifier could understand? And you could then load that file into the deminifier in your browser, and it could apply it in reverse during the deminification process?
You could debug production websites, using the production minified code, but actually be able to read it in view source, and even perhaps in Firebug! :-)