TamperData is a Firefox addon to view and modify HTTP/HTTPS headers and post parameters, trace and time http response/requests and security test web applications by modifying POST parameters.
TamperData allows you to see and/or modify the requests as they are sent to the website you navigate.
It is a useful tool to discover the structure and functionality of the web application you are examining.
You can pause a request temporarily, examine it, modify it if you’d like and then send it back.
You can identify hidden form fields, modify query arguments and request headers and inspect the response from the remote server of the web site.