Fiddler2 comes in. Fiddler is a web debugging proxy for any browser, system or platform. Which makes it quite handy if you're doing AJAX stuff on a lot of different platforms from Java to Node.js.
special with Fiddler is the composer feature which allows to make AJAX
verb (GET, DELETE, PUT, HEAD, etc.) calls to your server side API. It then allows you to look at the response headers, cookies, the raw results and a bunch of other values to help you debugged that code.
Use this with Mozilla Firefox to get the most mileage out of it since it does integrate into it quite nicely. Chrome doesn't do it because of that new rule that you can't install a plugin that's not found in the Chrome store.
It also has a nice set of add-ons which is just icing on cake. I like StresStimulus add-on which allows you to do load testing with your web apps.
Another thing is this thing is free and will run on Linux boxes (needs Mono though).
Add this to your toolbox.