Manage account order in Thunderbird

This is doable. In your user profile folder find the prefs.js file.
In this file change the order of your accounts like this:
user_pref("mail.microsoft.accounts", "account4,account1,account3,account2");

You can also use a Firefox extension called 'Folderpane Tools'.

Once you successfully installed the extension you will be able to define the new sorting orde.
To configure Folderpane Tools:

1. In Mozilla Thunderbird, open Add-ons from the Tools menu.
2. Locate the Folderpane Tools extension and click on it.
3. Press the Preferences button

Then, you just rearrange your accounts to your taste (and select the default start up folder you want to use).