I use Firefox, and there is a tool called Developer -> Web Console. It shows more information about GET/POST requests, which page we connected with XMLHttp, request headers and information like this. I'm sure there is a similar feature on Chrome too.
There is an AJAX call on a page and I have to see what it returns to the website. It is just a GET request and returns JSON. When I manually request that page (e.g ajax/view/my_purchases.php) it just shows a blank page, but when the website requests it with AJAX, I can see what content it returned in HTML.
Basically, What kind of tools I can use to see it? It can be a standalone application, Chrome or a Firefox extension. I'm okay with any.
