If you want to turn this behavior on in Firefox, you can do so with a simple add-on:
https://addons.mozilla.org/en-us/firefox/addon/omnibar/
but it's off by default because of the MASSIVE privacy implications.
Merging location bar and search bar doesn't mean you can't give URL higher priority or that you must query search suggestion every key stroke.
a) I can drop-down the list of available search engines and see what's available, without having to remember any key-words to do the searches.
b) (and this is the killer feature for me) If a particular search engine is selected (say, IMDB), I can right-click on any word on a page, and one of the context menu options is "Search IMDB for <selection>..."
Also, horizontal real-estate is not much of a concern on a wide-screen monitor - how much of the URL would I really want to see..?
I've been asking myself this question many, many times. You can hide the search field: right-click on the toolbar > customize > drag/drop the search inside the window > done.