There is no Confirm control - normally, after you rearrange the accounts order in Account Settings, the new order is immediately visible in the main Folder list. One account will be designated as the Default account and will always appear first in the list, but the rest should be able to be rearranged.

The order of the accounts is stored in the file prefs.js (found in your profile folder), in the line "mail.accountmanager.accounts". This file should be automatically updated with you rearrange the accounts and should not be manually edited, especially when Thunderbird is running. Can you, when Thunderbird is shut down, locate this file and tell us if it's date and time coincides with the last time you ran it? There are other things to check, but that will tell us something significant.

Hi Lin, many thanks. The prefs.js file seems to update regularly - last time it was refreshed was 5 mins ago. However if I try to rearrange the accounts again right now, it doesn't seem to update... There are four more prefs#.js numbered 1-4, which are six years old. When I close Tb and re-open the prefs.js is refreshed, but the wrong order persists. Any thoughts much appreciated