Rated 4 out of 5 stars

good to go so far - can test here...
http://www.xhaus.com/headers

maybe needs a toolbar button to toggle state?

i miss another ext which, in addition to the referrer, could also spoof the UA. would be nice to see a more comprehensive header manipulation tool that worked on a per-domain basis

This review is for a previous version of the add-on (0.0.3.1). 

About the state toggling: I made the extension restartless, so you can use the already present functionality in the add-on manager.

It will be there as soon as the new version is reviewed.