For the time being browser developer tools do not have a built in JSON pretifier in the network tabs. I use localStorage to persist many JSON objects as well. Again no visualizers. If you run your AJAX requests through Fiddler you are in luck because it does format JSON automatically. However it is not a proper editor.
Over the summer I have found myself using JSON Editor Online several times a day. This is a free online tool that makes JSON editing nice. The editor is a very simple tool, consisting of two columns. The first column displays a raw JSON object. The right column displays a tree/leaf view.
Once your JSON passes validation you can copy it to the tree view by pressing the right arrow in the margin between the two editor windows. This will transfer the object over. You can start editing the object now in the tree view. As you react stopping points you can copy the updated object back to the raw view on the left.
At the top of the layout is a menu where you can load a JSON file from either your local file system or a URL. This is a very nice feature because I do save many JSON files while I am working as well as pass them back and forth with other team members. This is also nice when you are working against a public API, like I do in my SPA book. You can also save the JSON you are working with in the editor. This lets you save the file locally. The editor does not persist your code in the cloud like jsFiddle, pasteBin, etc does.
I encourage web developers to bookmark this tool. Oh, I almost forgot to mention the tool works offline! Great for when you are offline, like me right now :). So the next time you need to edit some JSON, try out JSON Editor Online.