JSONView作者: Ben Hollis

Normally when encountering a JSON document (content type "application/json"), Firefox simply prompts you to download the file. With the JSONView extension, JSON documents are shown in the browser similar to how XML documents are shown. The document is formatted, highlighted, and arrays and objects can be collapsed. Even if the JSON document contains errors, JSONView will still show the raw text.
Once you've got JSONView installed, check out http://jsonview.com/example.json to see the extension in action!