If there is a Content-Disposition: attachment
reponse header, Firefox will ask you to save the file, even if you have JSONView installed to format JSON.
To bypass this problem, I removed the header ("Content-Disposition" : null) with moz-rewrite Firefox addon that allows you to modify request and response headers https://addons.mozilla.org/en-US/firefox/addon/moz-rewrite-js/
An example of JSON file served with this header is the Twitter API (it looks like they added it recently). If you want to try this JSON file, I have a script to access Twitter API in browser: https://gist.github.com/baptx/ffb268758cd4731784e3