My guess is your anti virus is scanning the file the mail for that folder is stored in. It is for that reason we recommend an exception in anti virus product for the folder where the Thunderbird profile folders are stored.

see https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data?#w_where-is-my-profile-stored

I have been using my anti-virus (AVG free) for more than a decade without any issues. Doubt this is the 'fix', but thanks.

You can at least give it a try to test. Once you have tested it, I mean really tested it, not just say you did, and the problem remains, then we can look at other causes.

Hi Bruce, oddly, without any 'tinkering' on my part, the issue resolved itself... at least for now. Thanks.

So a temporary glitch. Did it resolve itself after restarting Thunderbird or rebooting your computer?

I don't know. The issue persisted for ~ a week or more (2 weeks?) while I waited for it to resolve. During that time I restarted T-Bird & rebooted my computer dozens of times. Then, a day or so after posting my query, it started working normally again. Possibly related was that I found I'd been using the 32 bit version of FF and installed the 64 bit version in an attempt to remedy another issue (https://support.mozilla.org/questions/1161305) which I'm still working on.

It could have been your antivirus then.

When Thunderbird updated to 52.1.1, the antivirus didn't recognize it. Eventually the makers of the antivirus pushes out an update so the antivirus does recognize the new version of Thunderbird.

This is the #1 cause of problems with Thunderbird after it updates.

Marshalldoc said

I have been using my anti-virus (AVG free) for more than a decade without any issues. Doubt this is the 'fix', but thanks.

Are you aware that there is no AVG free as such anymore? See https://blog.avast.com/avast-and-avg-become-one

You might also not be aware that Yahoo has been sold or is in the process of being sold to Comcast. As Comcast have been migrating their own customers to AOL following their purchase of that company, there is a fair likely hood that you will end up with AOL mail

This is a time to pay attention, things are changing.

Very interesting! I've never seen any announcement from AVG of its acquisition by Avast (with whom I'm unfamiliar) and I'm still getting regular updates to my AVG Free (for free).

Was aware of the sale of Yahoo! to Comcast but not that they're migrating their customers to AOL.. which I'm not sure I'd want to use. I have a G-Mail account under a pseudonym that I only use occasionally and don't particularly like using both in terms of convenience and the Google association.

Any suggestions for a new e-mail client?

E-mail client = E-mail program. ie. Thunderbird.

Hopefully you mean you want suggestions for an e-mail provider.

Of these major ones that I have test e-mail addresses for (Gmail, Yahoo, Outlook, Yahoo), I've found that Gmail is the most reliable.

You can have more than one Gmail address if you want. It just means having to log out and log in to each one separately.

Thanks for the clarification on terms. Improves communication. May migrate to G-Mail if Comcast starts pushing me to AOL.

Marshalldoc said

Very interesting! I've never seen any announcement from AVG of its acquisition by Avast (with whom I'm unfamiliar) and I'm still getting regular updates to my AVG Free (for free). Was aware of the sale of Yahoo! to Comcast but not that they're migrating their customers to AOL.. which I'm not sure I'd want to use. I have a G-Mail account under a pseudonym that I only use occasionally and don't particularly like using both in terms of convenience and the Google association. Any suggestions for a new e-mail client?

My personal choice is GMX... they are governed by German privacy laws, so they are very different from the American companies that mine your every word for advertising sales.

Much appreciated!

Well, as if Murphy was listening in... the day after my last post (the 3rd) the whole issue began again. To my knowledge there's been no change in any aspect of my computer nor any AVG updates... or anything to explain it. But, after behaving just right since I said the issue had 'solved itself' my T-Bird is now failing (again) to sort messages (again, even after doing 'folder repair' and making new message filters - yesterday and the 4th). In addition, when I select the whole list of folders to mark "message read, only some are getting so marked, leaving me to manually mark as read all the rest. I just downloaded 250 messages and spent nearly 40 minutes sorting and marking them. Thoughts?

Try running Windows in Safe Mode with Networking, and Thunderbird in its safe mode both, and see if filtering works or not.

Restart Windows 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

If Thunderbird works OK when Windows is running in Safe Mode with Networking, but doesn't when Windows is running normally, then another process/program is interfering. The first thing to look at in this case is the security software on the computer.

Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled".

Or just hold down the Shift key when you start Thunderbird.

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 for the input. It'll take a couple of days to test the various permutations (Win in Safe; T-Bird normal; Win in Safe; T-Bird in Safe). Will advise.

Okay, so I've had the opportunity to download mail in both settings; FF safe/TB nl & FF safe/TB safe. Neither setting improved the sorting issue. I also tried the 3rd permutation; FF nl/TB safe, which didn't improve the issue either.

I'm open to further attempts to resolve the issue...

You say "FF", but I'm assuming you mean Windows. If you really mean Firefox, not Windows, then you still need to try Windows Safe Mode with Networking.

Assuming you did that, then what I recommend next is:

1. Run a Disk Cleanup then Check Disk. How to run a "Disk Cleanup" in Windows How to run a "Check Disk" in Windows

2. Create a new profile in Thunderbird. If the new profile works better, then import your stuff from the old profile to the new profile. Two different ways for the latter: http://brucejohnson.ca/ThunderbirdProfile.html http://brucejohnson.ca/ThunderbirdDataImport.html

Hi Bruce... You're right, I misthought FF instead of Windows.

Coincidentally my bi-monthly disk check & repair is scheduled for tonight so I'll be able to see if that's helpful by tomorrow night.

Creating a new profile looks almost as daunting as continuing as is & hoping for another 'spontaneous remission' as happened before.

I've been using MozBackup (no longer supported, I know) to backup FF & TB every other week. Would this obviate the need to do all the backup steps in your instructions? Last time I had to reinstall TB (~ 14 months ago) it worked well but did require folder repair on all folders and some other things to get back to normal; after which it worked very well for quite a while.

P.S. I'm very grateful for your prompt & thoughtful responses as well as for your patience.