Firefox has a built in JSON viewer, but it didn't always. JSONView was built to display JSON documents in an easy to read format, which is still nice if you don't like the default viewer. 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!
* "Save as..." saves HTML instead of the original JSON document.
* "Page Info" panel reports content as HTML instead of JSON.
* Copying and pasting from the formatted JSON does not copy the original document.