Excellent Rated 5 out of 5 stars
Kuddos for M Gareth Hunt, this extension is very simple and incredibly usefull!
(well, provided you need to MODIFY HEADERS in the first place ^_^ I can't believe the amount of noobiness, navel-gazing and arrogance combined in a few comments).
Just open it from the "Web Developer" Menu, clic "Help" and you're started. Takes less than a couple of minute if you know e.g. the headers you wanna modify :-D