That search feature is called the keyword service. To turn it off, try the steps in this article under "Turning off the web search in the address bar": Search the web from the Address Bar. Does that work?

Please note that some ISPs, instead of simply returning site not found, will send their own search results. One way to spot this is to change the search engine on your search bar to, say, Wikipedia, and see whether the failed address lookup then searches on Wikipedia. If not, your ISP may be sending the search results.

You can set the keyword.enabled pref to false on the about:config page.

• http://kb.mozillazine.org/about:config