Works for me on Windows 10.

Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Thanks, Matt-- I've discovered since that there seems to be a problem with my "Documents" folder indexing within Windows 10--even in File Explorer. So I need to solve that problem in Windows and see if fixing that solves it in Tbird. (It turns out that Tbird is sorting my other folders just fine--only Documents is a problem). I will post when I figure it out, but it's taking some time.

Actually, I just conducted another test. It does manage to sort Documents after all, but it takes about 30 seconds (same as in Windows File Exp). This is on a brand-new Dell Latitude with tons of RAM on a Core i7--but it didn't happen on the computer I migrated from, a 5-year old Latitude on an i5 with less RAM. Somehow the Documents folder got messed up in the migration, and Windows reprocesses/indexes it every time it gets read. I'm not sure how to find the culprit, but it's not a Thunderbird problem, so I'll close this thread. Thanks again for your reply.

I would suggest you run chkdsk on the hard drive., getting there, locate the C: drive in the windows file manager and right click it in the left hand pane. Select properties and then the Tools tab.

Check the drive for errors. This usually requires a restart and it happens in the text part of the startup sequence, but it will identify any allocation errors in your files, something I think you most probably have, hence the re-indexing all the time.

The check could be slow, depending on disk speed and the amount of data on the drive. Anything from a few minutes to half an hour is fairly common.

Thanks. No errors found.

I had also tried moving everything out of that folder to another, and deleting the folder, and creating a new "documents" folder. When in a different (new) folder, it sorts just fine, instantly. But when I moved it into the new "documents" folder, the same thing happens -- even with indexing turned off for that folder. It must have something to do with the Documents "Library" entry, which it seems I can't change. Even with only half the original number of files /subfolders, the problem persists. Quite a mystery.

Just on the off chance. Try this bit of Thunderbird troubleshooting. Lots of the inexplicable these days can be put down to security software, be it anti virus, firewall or some special program the bank give you to log into their web site. This is exacerbated by the Junk ware Dell insist on polluting their new machines with. In the consultant community it is considered necessary by most to wipe and reload the entire operating system with media not supplied by dell before they can "trust" the machine. Such is the distrust of some of the almost virus like pre-loaded software. Then they bundle a very questionable anti virus.

Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.