After the advent of Firefox 3.x, lots of people including yours truly, migrated to Google Chrome which was much faster and had a very low start up time. If you have been using Chrome, then you have surely gotten used to the single bar for both address and search. If you want the same in the all new Firefox 4, here is the solution.
If you do not have then, Download Firefox 4 Final.
The easiest solution is to get an extension to do the needful. This addon is called the Omnibar.
You can download the addon here.
Install the addon and restart Firefox 4, and you are set. The address bar and the search bar are now combined into a single bar, like in Chrome.