When trying to protect your privacy, the last thing you want is for a search engine to collect data on what is your search history behavior. That's called profiling.

Below is an abstract for the United States Patent Application 20050222989 "Results based personalization of advertisements in a search engine" filed by Google

Personalized advertisements are provided to a user using a search engine to obtain documents relevant to a search query. The advertisements are personalized in response to a search profile that is derived from personalized search results. The search results are personalized based on a user profile of the user providing the query. The user profile describes interests of the user, and can be derived from a variety of sources, including prior search queries, prior search results, expressed interests, demographic, geographic, psychographic, and activity information.

Most extensions and tools commonly used to prevent data profiling by search engines work by concealing information from outsiders. TrackMeNot takes the opposite approach and actually sends out a bunch of information for the search engines to process. It sends mostly false information, which means your real search activities remain hidden from view and search engines won't glean any useful data from your visit.

TrackMeNot 0.5.32
Works with Firefox: 1.5 – 3.0.*
Homepage: https://addons.mozilla.org/en-US/firefox/addon/3173

Description: Protects users against search data profiling by issuing randomized queries to popular search-engines.