In Firefox 4 by default the Stop, Go and Reload (Refresh) buttons are combined and attached to the right hand edge of the location bar.

When you are typing in the location bar it will show the Go button. When a site is loading it shows the Stop button. At other times it shows the Reload button.

If you want separate buttons, right-click on a toolbar and choose Customize, you can then drag and drop the stop or reload buttons and place them elsewhere. If you place them in the order "Reload-Stop" on the right hand edge of the location bar they will be combined again. For more details on customizing the toolbar see https://support.mozilla.com/kb/How+to+customize+the+toolbar

Ok, but when I try to customize the toolbar, there is no refresh button (or stop button) to choose from! All I want are "stop" and "refresh" buttons on the left, next to the arrows. You had them in previous versions.

The combined buttons should "split" (that's the best I can explain it) so you can drag them to where you want them. If you change the order, I think that prevents them from being recombined.