Puntuado con 5 de 5 estrellas
Solves a major problem (we should be able to do this from Thunderbird's native settings). I was so relieved to find it still works in the current version of TB (57), though installing it is somewhat convoluted, probably due to the legacy settings.
If you're having trouble, first you have to download the plugin file, then go to TB Tools/Add-Ons, select Extensions in the vertical menu on the left, then click the gear at the top of the page to access the menu with the option to "Install add-on from file."
For the person who asked how to disable it, you have to go to your extensions panel as described above and disable it, then close Thunderbird and reopen it. You will need to restart TB again when you re-enable it.
Yeah, that's a slight hassle, but not nearly as much of a hassle as accidentally dragging a folder with dozens of subfolders and hundreds of messages, which happens to me often!
Update 12-18-17: Still being maintained - just got an update to the current version of Thunderbird. If it was possible to deactivate and reactivate it without restarting Thunderbird twice, that would be perfect (I would pay for that).
Update 5/18/18 - I figured out a workaround to rebooting Thunderbird twice to disable and re-enable this plugin when you want to move a folder. Just rename the folder you want to move (I call it "delete me"), then create a new folder with the original name in the new location, then move all the posts to the new folder, then delete the now-empty old one. If you only have one folder to move, this is much faster than restarting TB twice.