Part of the new user interface design of Firefox 4, the reload and stop buttons have been merged into one button which is now on the right-side of the address bar.
For those who would prefer to have the two separate buttons instead of one, simply right-click on the button and select Customize…
Here you will see the buttons are separate while in the Customize mode. To keep them separated you need to move the refresh button so that is not first button to the right of the address bar or you can remove the stop button. You could also switch the order so the stop button is first followed by the refresh button. To revert back to the combined button you need to have the refresh button as the first button to the right of the address bar followed by the stop button. When you are finished click Done.
The buttons can also be split if under the Show drop-down menu in the Customize Toolbar window either Text or Icons and Text is selected. Personally though either of those options take away from the clean look and make the navigation bar very cluttered looking.
