Ok this comes under the "stupid" or "obvious" question heading: I have both Firebug and JSONView installed in Firefox. I can see the JSON in Firebug but it isn't formatted nicely which is why I installed JSONView.
Trouble is, I haven't a clue how to get JSONView to work its magic i.e. bring up a browser page with some nicely formatted JSON data of my choosing.
Again, I'm sure this is blazingly obvious but I just can't figure it out.