How can I put my e-mail folders in alphabetical order?

With your version, the only option is to manually rearrange them by drag-and-drop, which can be tedious and error-prone. Version 150 and newer have the new option to Reset Folder Order, which will resort them in alpha-numeric order. That option is found when you right-click on the account name in the Folders List.

Thanks. The drag and drop method (which I've tried) does seem error prone. According to my Thunderbird, Version 140.11.1esr (64-bit) is the latest version. When can we expect Version 150? Thanks.

The latest "release" version is 151.0.1, available at

https://www.thunderbird.net/en-US/thunderbird/all/

The esr versions won't offer to update, you'd have to install the Release version over the top of it. If you previously installed it from a 'store' (Windows Store, Snap, Flatpack, Apple) proceed with caution, check if they offer the release version yet. Also, make sure all your add-ons are compatible.

Thanks! I've upgraded to the new Release version! And, I was able to alphabetize the folders!