Tamper Data is a great addon for Firefox browser more for system and network security professionals and network admins and certainly to a degree for network application developers.
Tamper Data add-on can simply be used as a packet capture tool for the traffic iniated from the browser for analysis and troubleshooting. You would realise this especially, if you are network administrator with proxies. You can graph the request/responses and export to an XML file.
But, the actual advanced utility is that it can actually intercept web requests and you can tamper the request headers. This allows you to test the website vulnerabilities. You can induce Cross site scripting, SQL Injection or edit the User-Agent paramters and more.
Click here to install TamperData add-on to Firefox. Once installed, start Tamper Data from the Tools Menu. This will start capturing the web sessions from the browser. If you choose test sites, click “Start Tamper” on the Tamper Data window.