Firefox has a separate field for search queries that is in the nav bar like the location field (terminology? I mean where the URL goes) is (and like the back button, the refresh button, the "downloads" button, etc, are) but is not combined with the location field.

At least my Firefox does, and I don't think I customized that particular aspect of Firefox.

I seem to recall someone here saying that Firefox decided not to combine them (location and search fields) for privacy reasons. In particular, the user can be sure that anything typed into the location field is never sent to Google's autocompletion servers.

I could be wrong about the things I said in this comment, though.

I'm not sure how this would pose a problem. I'm not the parent commenter, but I don't typically go to Google to search for sites I frequent. It would be such an odd thing to do—granted that I know the URL—that I would be forced to consider what I was doing even more vividly than if I had to type the URL out.

Unless you mean the search bar?

  browser.search.suggest.enabled = false

Or, if you mean that you'd like to have autocomplete on for searches, just leave the default:

  browser.search.suggest.enabled = true

This does not affect the URL bar.