I think it's the same button, actually. While a page is loading it display an "X" and after it loads, it displays the reload circular arrow.

For others reading, I'm referring to the very end of the box containing the URL. From left to right:

site-info | URL | bookmark-star | history-drop-arrow | stop-reload

Firefox 4 uses a combined Stop/Reload/Go button that is positioned at the right end 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.
You can middle-click that 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 "View > Toolbars > Customize" window to move the Stop and Reload button out of the location bar.
• 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.

Great! Just what I needed. Pity this info is not in the "normal, documentation named 'Help'"!