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

You can also press F5 to reload a page.

Unfortunately, i cannot find the reload button in the screen which appears when I try to customize the toolbar.

If you click on the tool bar - you should see a circle arrow to the right of your address bar. Just drag that to your toolbar - and voila you have your refresh button back!