Pieces keep fitting in for the Firefox 4 theme refresh. Recent development nightlies now feature the planned stop-reload-go button next to the location bar.
When the page is loaded, it becomes a reload button.
A stop button while loading.
And , if you edit the web address, you get the go button.
While it should make more sense to new users, this is no doubt going to anger more than a few long time Firefox users used to clicking next to the back button to reload or stop the current web page.
However you can take the buttons back to their previous location through the Customize Toolbar window: open the Minefield (Firefox) button, and select Toolbar Layout… in the Options menu. The buttons are decoupled and you can drag them at will.
You can also take a half-way approach by moving the unified button to the left of the location bar with this user style. You will need the almighty Stylish extension.
UPDATE: It is actually possible to decouple the reload, stop and go buttons, as noted in the comments. The miserably wrong statement has been updated to reflect this. Thanks to all commenters!