Well, you seem to have mistaken a bit. Search engines are not built into browsers. Basically, when you enter search terms in the search bar of the browser, the default search engine website (google/msn,yahoo) is used. So the ads you mentioned were probably from the search engine website and not the browser.
So, all the three browsers you mentioned are ad free, Firefox has lots of plugins to aid and Google chrome is a very fast browser. Although, the latest version of Firefox 3.5 is a lot faster than previous versions, but still if you want speed with minimum functionality of a browser, go with Google Chrome.
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