Firefox is one of the most used web browser and its main reason is that it provides lots of add-ons which can make your work easy.
Well most of the browser provides separate Address bar and Search bar, but if you look at Chrome it has only one bar and you can make search from it too.
If you want same thing in Firefox then Omnibar is the best Add-on for you, It will combine both Address and search bar into one.
It provides a clean and nice interface.
You may use following link to download this add-on :-