There is a beta version: https://github.com/protz/Manually-Sort-Folders/issues/199#issuecomment-1641133891

Thank you for the beta version. I don't understand, however, how downloading this version is supposed to sync with Thunderbird. What do I have to do after downloading it? In other words, how does it become a Thunderbird "add-on?" Thank you.

If it's zipped, you need to rightclick and unzip. in thunderbird, click tool>addons and click the gear wheel to install from a file.

Sorry for being dense, but what am I right clicking on? When I download, there are multiple files and right clicking on them doesn't show an unzip option.

That link pointed to a zip file, so I'm not sure what you downloaded.