Only half the solution, to be sure - as has been commented before. Excellent on that half, but the other part is sorely missed.
So, if I am at a google search page, and click on a link to whatever.com, then this addon lets me control the referrer sent whatever.com. It does _not_ allow me to control all referrers sent to any site that I might link that search page at google.com. That's a headache. Some of my sites will be able to see my google search results and some won't, depending on whether I've coded them in to the addon. The only solution is to use the global default to block or uniformly spoof all referrers (at least to 3rd parties). Then, that default can be modified for specific sites.