the only way to rearrange Containers is to edit "containers.json"
when I click on Firefox Multi-Account Containers Extension button and then click on Manage Containers, it shows an option to arrange the containers but that arrangement only shows when you click on the extension button. if you right click the NewTab button (+) the container is listed as before.
I suggest when the user modify the order of Containers that it would also reflect everywhere in firefox and most importantly on the New Tab button.
Also, Separators (like in Firefox bookmark folders.) Separators, would be an additional way group multi account containers along with folders.
Lastly, not for me, but many suggested this, which is to add a way to rearrange the Containers alphabetically.