I was looking for replacement for JSONview that stopped working in FF 57.
This one looked fine to me, though, it sometimes feels overloaded with all those guidelines and size info.

The other problem I've found, it doesn't work with "data:" protocol, so I can't use the extension to prettify big one-line JSON. So, typing something like:

data:application/json,{"test":"test"}

in the address bar just gives me the one-line text. Hope, this could be fixed.