I think you misunderstood my example. It doesn't autocomplete domain names, it shows search results. For example, type "Hacker" and the top match might be Hacker News. Type Newton and the top hit might be the Wikipedia page for Isaac Newton. And so on. Safari searches in a bunch of sources, not just Google.

Firefox, on the other hand, only shows you searches. It might offer "Isaac Newton" as a search suggestion, but that just triggers a search. It has no knowledge of other sources of information (beyond bookmarks and history).