Normally firefox will pop up and ask you to view application/json as a file (like a file download). This can get annoying.
To view this in Firefox 3 Directly you can use ‘Open in Browser’ add in:
or the JSONovich is a Firefox extension that pretty-prints and colorizes JSON content.
To associate it with an external application directly you can use Firefox ‘MIME Edit’. It word with Firefox 2. You can get it from: https://addons.mozilla.org/en-US/firefox/addon/4498
You can do the same in IE without needing a tool by updating the Windows Registry as follows:
Further info at: http://msdn.microsoft.com/en-us/library/ms775148.aspx