I'd suggest you check your disk. When you see a 'Repairing disk error' message, that certainly isn't related to Thunderbird, but it sounds disturbing. Wrt the blue screens, make sure all your drivers are up to date.

Maybe not, but why did the Thunderbird profiles disappear at the same time? And nothing else affected? And why was Thunderbird not right after the other system crashes I've had? All drivers are bang up to date. There has been a clutch of strange happening reported with Tbird on 8.1. And at least one other report almost identical to mine, with apparent disk errors. I suggest that might not be simply a coincidence.

Thunderbird is sitting there open with dozens of files open when your hard disk decided to turn turtle, which other program has dozens of files open. Probably none.

What other program is undertaking regular activities on open and recently used files ... Anti virus. The number one cause of computer problems.

Yes there have been strange happenings with windows 8.1. But I think a large part of that is windows, and defender. Not Thunderbird. The windows shadow copy and the frequent copies made to support the system restore I think are all causative in the problem. The operating system is moving and copying files while they are in use in some cases.

Do I have definitive answers? NO! I probably never will as I have bypassed what I see as another Windows ME Virus released from Microsoft. So I will never have a machine to experiment on. Unless Windows 10 is a wonder my Windows days will end with this PC after almost 30 years

But be assured, disk errors are not created by any application software. Application software runs in ring 3 of modern operating systems. To access the disk at a low enough level to cause errors the software must run in ring 0 or real/kernel mode. That leaved drivers, the operating system and anti virus programs as well as some low level disk utilities as a cause, unless the disk itself is failing. Even new hardware can fail. What does the SMART report for the disk show?

OK Matt, your argument is compelling! I'm probably too ancient to try and move away from Windows to an alternative or to a Mac, so I guess I'm going to have to keep living with this stuff. Email is really important to me so perhaps it's time to do it all in the cloud. Thanks for the time you've taken with this, I shall take a proper look at my hard drive!

One further point, please no offense intended.

I lost access to my blogger account with Google several years ago, automated spam clearing bot of Google just deleted everything. Months passed before I managed to actually get my data restored. My point here is I now simply do not trust the "cloud" with my data. They make no substantive guarantees as to privacy, nor availability on a day to day basis or in the longer term. So I am wed to a local copy as well as a cloud copy. I think they cover different loss scenarios. My local copy might get wet or burned, the cloud might get hacked or the company just loose the data.

Some suggestions I can make that might help long term is create an exception for the Thunderbird profile folders in your anti virus program. They do not need to be scanned every time you move or delete a mail.

Turn off incoming mail scanning. Especially if your are using Nortons. Most Thunderbird users are using SSL or TLS. Most ISP's and free mail providers now require it. Nortons does not scan email over encrypted connections. So the functionality can only have a downside.

OK, I use free version of Avast, and I've excluded the profiles. Not sure about stopping scans of incoming, though ! Cheers, Dick.