JSON Lite от Lauri Rooden

Disable Firefox built-in JSON viewer first - go to about:config page and set 'devtools.jsonview.enabled' to false

Features:

- Format JSON/JSONP responses and clipboard
- Select and format/encode/decode text on every page
- Collapsible arrays, objects and long values
- Collapse all childs with Ctrl(Command) + Click
- Collapse property also on all sibling items with Alt(Option) + Click
- Count items and properties in a collection
- Show collection sizes
- Do its best to show large (10MB+) JSON files
- Do its best to show invalid JSON files
- Do not sort objects with numeric keys
- Do not round large numbers
- "Save as..." saves the original JSON document
- Select and copy a working JSON from the formatted JSON