Firefox 4.0 has a combined Reload and Stop button that appears at the far right of the location bar.
- Open the "View > Toolbars > Customize" window to move them out of the location bar.
- Swap the two buttons or move them to their previous position at the left of the location bar.
- If the order is "Reload – Stop" to get a combined "Reload/Stop" button.
- Swap the order to "Stop – Reload" or separate them otherwise to get two distinct buttons.