Thanks to Ashish Datta, Firebug is having a fresh new support for inspecting JSON in net responses. This feature allows to see JSON formatted as an expandable tree of items and also explore them using Firebug's Dom tab.
The view is available within Net panel and visible as soon as a JSON request is expanded. In such a case the user can see another tab (like Params, Headers, etc.). See the following screen-shot.
The feature should be part of Firebug 1.4a11.
Ahshis is a senior at Tufts University and he did this work as a part of his class focusing on open source development. It's great to see how community around Firebug is growing and willing to contribute to this indispensable tool!