Useful (especially since firebug 1.5 crashes firefox on linux x64), but weird behavior on values of type string with line feeds inside (and possibly the other control characters which are escaped in json, range U+0000 through U+001F - did not test).
Bug occurs when line feeds are escaped with the two-characters sequence %x5c %x64, not the six-chars sequence U+000A.
Symptom: display is correct until the line feed, then the rest of the json doc is not displayed.
I don't know what people expect as display for line feed (just ignore it?), but the rest of the document should be displayed anyway.
As a suggestion, there's another json addon out there that displays line feeds as the ascii representation of the escaping sequence, "\n"...