TB is the best tool to access and manage multiple mail-accounts, and with TB91.0 came the ability to rearrange the order of accounts , BUT: only the order of folder in the main folder pane is affected.
Plug-Ins like Identity Chooser make it really convenient to work with a handful of mail addresses. Still, if you use a bunch of identities (or just still have a load of legacy ones), it becomes confusing—or let's you scroll for ages.
It could be simple: buttons in the settings page, for ordering the accounts, to not fuzz around in some config text files to achieve the result.
See attached screenshot for a quick mockup