In this file change the order of your accounts like this:
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:
- In Mozilla Thunderbird, open Add-ons from the Tools menu.
- Locate the Folderpane Tools extension and click on it.
- Press the Preferences button