Network monitor - browser extension.

Network monitor - shows requests and responses, including the HTTP headers, of the current page, or globally.

It is possible to use it at any time, also after the page has loaded.

This is useful when there are random errors in some cases and we just want to see what happened, see a list of the past requests.

It also displays request count in current tab.

Installing

Firefox

Available on AMO: https://addons.mozilla.org/en-GB/firefox/addon/netmon/

Chrome

Available in Web Store: https://chrome.google.com/webstore/detail/netmon/fnbdjaofejpjedhleannakpdpifmchlo