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

I can not find these buttons in the customized tool bar? please help as i miss them

By default they will not be in the customize toolbar window, they will still be on the toolbar. When the customize toolbar window is open, look to the right hand side of the location bar, the icons should be there, you can drag and drop them elsewhere.

Thanks, the 3rd reply was helpful. But why on earth could the icons not have been left where they were? (If it ain't broke, don't fix it.) I have wasted half an hour of valuable time trying to find this out.