Acesse about: config e defina as seguintes preferências para false :
browser.fixup.alternate.enabled
True (default): Try to fix up
http://foo
tohttp://(prefix)foo(suffix)
False: Do not second-guess the user
keyword.enabled
Determines whether to use keywords
True (default): Send non-URLs entered in the Location Bar to the URL specified in keyword.URL
False: Disable keywords
Onde a documentação para keyword.URL diz isto:
URL to use for keyword lookups. Default value in Firefox 2.0 is
http://www.google.com/search? ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=