This is exactly how Firefox currently works, but you just press enter instead of up and then enter. If what you've got in the address bar doesn't look like an address, it'll google it for you.
It'll actually use whatever you have the search bar set to.
Basically, when you type something in the address bar it would show your history and add "Search for ..." as the last option. So, even if you had large history, it was enough to press the UP arrow and switch from address to searching.
AFAIK, this is still the way it works on SeaMonkey.