You can do it by going to the profile folder and editing the containers.json file with closed Firefox.

But try to not break things, at least save a copy of this file. Don't change any numbers in the file, only change order of objects in the structure of json.

Thank you TyDraniu! I just also found that by clicking on the Multi-Account Containers button, then click in the "Manage Containers" line at the bottom of the pop-down list, then click and hold on an individual container's name, its position can be moved up or down the list wherever you like.