livehttpheadersLiveHTTPHeaders is a Firefox plugin for viewing and manipulating HTTP headers. From the site:
The goal of this project is to adds information about the HTTP headers in two ways:

  • First by adding a 'Headers' tab in 'View Page Info' of a web page.

  • Second by adding a tool in the 'Tools->Web Development' menu to be able to display http headers in real time (while pages are being downloaded from the Internet.

  • Third by letting you edit request headers and replay an URL (beta). Look for the Replay button in the live window!

This project may be of some help for the following:

  • Help debugging web application.

  • See which kind of web server the remote site is using.

  • See the cookies sent by remote site.

I've found it to be handy when creating performance test scripts or when trying to debug a script.