Not as full-featured as its Google Chrome counter-part, which has crossed the version 1 boundary and it seeing development still. But it did exactly what I needed it to do: temporarily spoof my X-Forwarded-For header in order to test a load balancer service I was working with.

For people who just want to set a static temporary value for their X-Forwarded-For header, I would definitely recommend this plugin.

Fifth star ready for the taking, if the latest version was to be pushed to AMO.

Works as advertised but I think it would be nice if you could define a whitelist of domains it should work for so it didn't send modified headers to all domains.
It would also be nice with an easy way to turn it on and off with a single click.