Tools > Options > Tab: Advanced > Tab : General > Config editor : “I’ll be careful, I promise!”
search for “account”
edit the variable “mail.accountmanager.accounts” and change your order here
quit thunderbird
launch thunderbird, change are now effective
You find the identities when you look for “mail.identity.id<n>.useremail” and the relation between identities and accounts looking for “mail.account.account<n>.identities”. If you have several identities for an account, you can’t influence the order of these or order them separately; you can only have these identities together in a different position.