It is possible to arrange them using an add-on, but that depends on which version of Tbird you're using. Go to Account Settings, select the default account, click the Account Actions menu at the bottom and select Set as default. This will move your default account to the top of the folder pane.

Thanks Stan your suggestion worked for my default account

My TBird version is 68.12.0 (32-bit) I have additional accounts I would like to sort. Which add on do I need?

The add-on is called Manually sort folders

Thanks Stan I will try & find that add on