QUick update to my previous response, since I only discovered it today (disclaimer, I worked on fra stuff for many weeks and haven't played with API dev for a little while)
Quantum has a built-in json viewer which is really nice, no extension to add (and it looks better than the ones I used in Chrome and Canary, although I haven't played with it yet to see how it behaves with large JSON documents). HAving this directly built-in, with filtering and a nice treeview is definitely a welcome addition imho.