The drag-and-drop function for moving tabs in Preferences > Customize Tabs does not work for people with large site membership. At Stanford, we had complaints from department admins who were members of hundreds of sites. I can't say for sure the number sites that causes it to go from slow to impossible to use. Loading this page and attempting to move tabs generates an error:
"A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.
It is also difficult to move a site from the bottom of the list to the top when the list is long because the page doesn't scroll automatically when dragging a site. Let me know if that should be logged as a separate issue.