Review for Manually sort folders by Neil Katin
Rated 5 out of 5 stars
Does what it says it does.
For those folks who are having trouble finding the "tools" menu: its *not* the hamburger menu; its the old "tools" menu in the window header.
To get it to show (on windows) press and release the Alt key in Thunderbird; the top level File / Edit / View / etc bar will show at the top of the window. You want Tools -> Manually Sort Folders.
To create your own collections, you must have a Mozilla Add-ons account.