Live HTTP Headers
Adored by web developers, this Firefox module lets you to examine the HTTP headers exchanged between the browser and the web servers.
Ideal for debugging, it will save you a significant amount of time by providing you with the exact time on what was actually sent by your applications (PHP, Python, .Net, etc.) to the browser. HTTP response codes, cookies, expirations, Cache-Control, Keep-Alive and all other headers are present.
Choose a simple solution that works even with secure pages (SSL/TLS).