Type about:customizing<enter> in the address bar. In the new window, look for the icon. When you find it, hold down the left button on it, and move it to where you want it to be. You can move any of the icons you want from here, but some icons are locked in place.

There is also a Restore Defaults button at the bottom middle.

You can also try to reset the browser.uiCustomization.state pref via the right-click context menu to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config