When something inexplicable like this happens, whether it is in Thunderbird or elsewhere in Windows, my first suggestion is to root the computer.

If the problem still exists after rebooting, then I suggest running a Check Disk to check for, and fix, any file errors on the hard drive.

I have in mind a "proper" Check Disk which will require a reboot.

For Windows 10: Click on the Cortana search button on your task bar, just right of the Start button (see first snip below), type cmd in the search box, but don't press Enter. Above that, right click on Command Prompt and choose Run as administrator. User Account Control will ask if you are sure you want to run it. Click Yes.

Then you will see a window similar to my second snip. In that you type: chkdsk c: /f It will then ask you the same question as you see in my snip. Type "y", then the Enter key.

Then close that command prompt window, and tell Windows to restart. It will do the disk check before Windows fully starts. Let it.

When I enter "cmd" the "Best Match" is Cmd Prompt. If I click on that the MSDOS window opens. I don't get a "Run as Administrator" choice. It expects an MSDOS command.

I see that I forgot to add my screenshots. They are below.

I also didn't emphasize strongly enough that you are to right-click on that "best match" Command Prompt, then choose "Run as Administrator"

Then you will see a window similar to my second screenshot below. In that you type:

chkdsk c: /f

Then press the Enter key on your keyboard. It will then ask you the same question as you see in my second screenshot. Type "y", then the Enter key.

Then close that command prompt window by clicking the "X" in the top right of the window.

Now tell Windows to restart (see my third screenshot).

It will do the disk check before Windows fully starts. Let it.