Originally posted by Andrew Benton
Use a newer build of Firefox. This is fixed in the Deer Park branch
Yes, it's useful and you do it by dragging a tab over to the left or right, then its new position is where you released the mouse button. Let's say you are a webdeveloper and want to compare a local site with one thats remote and you opened some tabs with other sites. Now you can drag the tabs position so it's easier to switch between them.