Rated 5 out of 5 stars
Five stars for this awesomeness.
I tried a similar extension for Google Chrome, but only found something incredibly unstable with bad handling on top of it. Only Opera included a HTTP header sniffer in its Dragonfly developer utility, but since that version of the browser won't receive any updates anymore, Live HTTP headers looks to be the only solution that works conveniently from within a browser.
Great stuff, I'll start using it more and more in the future as I phase the old Opera out.