How to get the actual URL a post/submit form redirects to?

Hi,

sorry if my question is not properly formulated but I dont know the exact term. As you may know when you submit data through a form you are actually accessing an URL with your submitted data included in the URL as the query strings. For example, if the form asks you to submit your name, age and job when you press submit you are actually being redirected to the URL below:

http:testsite.com/form.php?send=profile&name=john&age=23&job=engineer

My question is how do I get this URL from any form? I used to had a Firefox extension that could do it but I forgot the name and I cant find it anymore.

Thank you.