Firefox 4 uses a combined Stop/Reload/Go button that is positioned at the right end side of the location bar.
During the page load process it shows as a Stop button and after the loading has finished the button is changed to a Reload button.
If you type in the location bar then that button becomes a Go button.
Middle-click the Reload button to duplicate the current tab to a new tab.

To move the Stop and Reload buttons to their position to the left of the location bar you can use these steps:

• Open the Customize window via "View > Toolbars > Customize" or via "Firefox > Options > Toolbar Layout"
• Drag the Reload and Stop buttons to their previous position to the left of the location bar.
• Set the order to "Reload - Stop" to get a combined "Reload/Stop" button.
• Set the order to "Stop - Reload" or separate them otherwise to get two distinct buttons.

• http://kb.mozillazine.org/Toolbar_customization

Unfortunately, those buttons don't even exist inside the customize menu when using on my mac

Open the Customize window via "View > Toolbars > Customize"

• If a missing item is in the toolbar palette then drag it back from the Customize window on the toolbar
• If you do not see an item on a toolbar and in the toolbar palette then click the "Restore Default Set" button to restore the default toolbar set up.

See also:

• http://kb.mozillazine.org/Toolbar_customization

If the above steps didn't help then see:

• http://kb.mozillazine.org/Corrupt_localstore.rdf