You can clone with
HTTPS or Subversion.
It would be nice if JSON View provided a way to submit JSON content "manually" for formatting (instead of just autoformatting JSON resources).
I would see this as a JSON View toolbar icon, that when pressed would open chrome://jsonview/format where users could paste/write JSON content or submit an url and push "format" to have it formatted (much like in this site does)
Why not just use that site, instead of JSONView?
Well, I can think of a couple of reasons (though I don't garantee that they make sense! eheh):
1. I like to have one tool for each "job" (in this case JSON View would be the "json" tool)
2. Using a XUL interface would allow for a nice, more user friendly interface
3. If I clean my cache and cannot get online I cannot use that site, whereas if JSON View had this feature then I wouldn't need to be online to use it
Can't think of anything else really, I guess it would be a "nice-to-have" feature =)
You can always name a file mystuff.json and drag it into Firefox to have it viewed as JSON. Otherwise, use an online formatter.