Live HTTP Headers is an extension for Firefox that can be used to view the HTTP headers sent and received when a webpage is loaded. This is useful, for example, to determine what the values in certain header fields are.
Installation requires a reboot. After that, click on the extension icon to open its recording window. Visit the webpage you want to monitor and you can see the HTTP headers logged in this extension window. You can save all the headers to a text file to analyze them.
Tried with: Live HTTP Headers 0.17.1, Firefox 41 and Ubuntu 14.04