Tested linking to https://www.whatismyreferer.com/ from a Google search, 23rd April 2017.
Setting rules to send the normal referer, a forged referer, no referer and a custom text string all worked as they should. Toggling the extension off from its toolbar icon resulted in the normal referer as expected.
Other active extensions were HTTPS Everywhere, uBlock Origin, and NoScript.
Not a WebExtension and thus incompatible with Firefox 57+. Suitable alternatives, including more general purpose header control extensions, may be available; look for something updated more recently than March 2016 when WebExtensions were added in Firefox 45.
My irritation with the people who allowed "HTTP referer" to be codified without it seeing the light of spellcheck continues to grow.