Review for Live HTTP Headers by Jason Barnabe
Rated 4 out of 5 stars
A very useful tool for developers. It could use some easy-to-activate features like only listening on certain tabs, or quick toggling for types of requests (when you only care about HTML and not images and the like). It unfortunately doesn't allow text selection of the headers, either.
This review is for a previous version of the add-on (0.13.1).To create your own collections, you must have a Mozilla Add-ons account.