Basic JSON Formatter од Dr. McKay

Simple addon which adds a button to the address bar for JSON content to pretty-print (format) it. To reduce clutter, the button is only present when the currently viewed content has a Content-Type of application/json or if the page content is valid JSON. Doesn't add a fancy viewer or colors or collapsing or anything.

If you want this to work as expected, disable devtools.jsonview.enabled in about:config.