Mozilla's placement of Google as the default search engine in Firefox makes a huge difference in Google's continued position as the default search engine people turn to. Google at this point doesn't provide profoundly better search than anyone else; several other search engines exist which have results more or less as good as Google's, or possibly even epsilon better. Google has three things going for their search engine: the inertia of people using what they've gotten used to as long as it remains "good enough", the pile of other services Google offers that integrate very well with each other, and Google's position as the default search engine in most browsers.
I think it's more of Google practically being ingrained into people's heads as the way to get somewhere on the net. Time and time again I've seen people with MSN or Bing as their homepage, and they'll just search for "google".
