A large part of the reason Firefox chose to keep them separate is for privacy. Everything you type into the search bar is sent to Google--even before you hit enter--for autocomplete purposes. The Firefox team felt that many privacy-conscious users trusted that the URL bar doesn't send info until they hit enter.
Type a search string into a 'universal' field and hit enter = perform a search.
Type a URL into a 'universal' field and hit enter = go to that URL.
Use some refined heuristics to determine the intention (ala the AwesomeBar in Firefox) and you've got the best of both worlds.
I don't use Firefox/Chrome daily, so I may be missing something, but I don't see why a dual address/search field needs to communicate with Google to be combined.