You may want to look into this -- it might be another add-on or toolbar you have...

demetris · on Dec 20, 2011

No. It is not an addon or toolbar; it is what happens when you select Amazon.com in the search box and then search for something.

The query string includes this: tag=mozilla-20

(To be sure, I just checked this in three different instances of Firefox: Iceweasel 9 on Debian, Firefox Nightly on Debian, Firefox 9 on Windows.)