You have some issues to solve this problem:
1. If target server service JSONP use it. You only have to add param callback=? end of url. "getJSON()" jQuery request add callback parameter automatically.
3. Not recommend but could issue which working is try to some older browsers versions where origin policy is not so tight.
4. This issues still working on firefox:
Do your cross-domain json request in browser and copy-paste response for file ex. "name.json" and save. Now you can locally get this files working (in jQuery: $.getJSON('name.json')).
Hox for PhoneGap developers!
If you made apps with PhoneGap, you can easily develop your app with one trick. Let see this guide: http://docs.phonegap.com/en/2.2.0/guide_whitelist_index.md.html#Domain%20Whitelist%20Guide