Rated 5 out of 5 stars

This worked for me:
1) Delete the bad Thesaurus - Reference.com search engine from you list of engines.
2) Go to the developer's website here: http://gooeysoftware.com/mozaddons/search/
3) Left-click the Thesaurus - Reference.com link. This should automatically add a new Thesaurus search engine to your list; with the correct URL pointer.