The Web Developer extension adds various web developer tools to the browser.
FoxyProxy is an advanced proxy management tool that completely replaces Firefox's limited proxying capabilities. For a simpler tool and less advanced configuration options, please use FoxyProxy Basic.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox...
View HTTP headers of a page and while browsing.