HttpFox is an HTTP sniffer add-on for Firefox by Martin Theimer. The other free and paid HTTP headers sniffing tools are here.

Once you’ve installed the built-in sniffer, just activate it by Shift+F2 or Tools -> WebDeveloper -> HTTPFox. The plugin interface is quite simple, having all the features accessible at once:

The extension gives the common set of HTTP sniffed data:

• Request and response headers
• String parameters
• POST data
• Cookies
• Content body

The timeline is not present, but the sniffer accomplishes the most needed sniffing tasks, results being subject for simple filtering.