Your web history, which is what the address bar searches in Firefox.
In Chrome these two aspects (history and Google search) are blurred, but it's not better for it. I'd say it is a good example of 2 half-assed features not making up for 1 good one.
If you want to search your Web History explicitly, just hit Ctrl-H. This is a case of personal preference. I rarely search my Web History, so I want my Omnibox real estate to be web search. Some people will prefer history ranked above search, others will prefer it the other way. Chrome should give you a preference setting for it.