My answer assumes you are referring to the Firefox desktop browser. What you need to do is right click in any of the empty spaces on the navigation toolbar or row of tabs at the top and click 'Customize...' (wish it wasn't spelt with a 'z'). In the new window you should find the icon named 'Back/Forward' which you can drag and drop into the navigation toolbar. Close the toolbar and the back and forward buttons should be back (in their usual style). If this hasn't made sense then it sounds like your either in full screen mode (f11 to toggle) or you have the navigation toolbar hidden; this can be unhidden by right clicking an empty space on the row with the tabs on it and clicking 'Navigation Toolbar'.