https://support.mozilla.org/kb/cannot-receive-messages

I changed the settings in both Thunderbird under AppData and in F-Prot. I rebooted, then compacted the files. No change. I still get the same heading under Inbox as under the general heading for that e-mail service. I rebooted in safe mode, and the compatibility screen came on and made changes in Thunderbird, saying afterwards that it had been incompatible but was now fixed. In safe mode, both the cursor "in-process circle" and the circle on the tab continue indefinitely when I try to click on the inbox.

I changed the settings in both Thunderbird under AppData and in F-Prot.

Please explain in detail what you did.

I was sent an article on not being able to receive messages--not our problem--but it mentioned anti-virus as a possible cause. I also found a help article which referred to conflicts with F-Prot, and it listed three changes to make but referred to a much earlier version of Thunderbird (3?) In F-prot I created an exclusion for it not to scan e-mails. In Thunderbird I was told to deactivate Global Search and Indexer under tools/Options/Advanced Configuration and a second change, which I do not recall and can't seem to find the same article again. I mis-spoke; no changes were made manually in AppData; they were all in Thunderbird or F-Protect through their interfaces. The problem concerns ONLY the in-box of ONLY one of six e-mail accounts in ONLY one of the three Windows 10 computers we use with the same software and versions. I just transferred my Thunderbird files with no hitch to a new disk in one of the computers, using Mozbackup. Should I try uninstalling Thunderbird on her computer and reinstalling?

Should I try uninstalling Thunderbird on her computer and reinstalling?

To achieve what?

She cannot see her Inbox messages. Sometimes it displays a Thunderbird page of text, sometimes you see lots of colored arrows. Neither new nor old messages show up. At the moment I only get a spinner circle at the cursor when I clock on the Inbox for our personal Yahoo inbox (her school Yahoo box is normal).

It's possible that mail files got corrupted. First of all create a backup of your Thunderbird profile folder. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Then try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a backup done prior to the corruption.

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

I followed instructions to back up the profile and found that it has trouble reading about half-way through the Inbox file. (Don't know if that is Inbox.msf or Inbox.) Stopped at same place four times, so allowed it to skip that file and back up everything else. I deleted the Inbox.msf file according to one set of instructions I found, but until now Thunderbird has not been able to rebuild it. Perhaps because there is a bad sector on the Inbox file? We will take the computer to Geek Squad Monday and have them check the disk in case it shows evidence of immanent failure. One curious thing that I have noticed. When I use right click on properties and then work down to the menu that has repair, our personal Yahoo account that has no problems shows no location for the Inbox files. I have tried cutting and pasting from one of our other Yahoo accounts (planning to change the number between Yahoo and .com), but it won't paste. I have tried to write the address in; I get the bar in the window, but it doesn't register any keystrokes. We have Thunderbird with the same five accounts on two other computers, so would it make sense to copy the inbox files from another one for that one account?

We will take the computer to Geek Squad Monday and have them check the disk in case it shows evidence of immanent failure.

If you suspect a disk problem Spinrite can do wonders sometime. https://www.grc.com/sr/spinrite.htm Note, it isn't free.

Before that I'd probably try to disable anti-virus software and try again. Better yet, start Windows in safe mode. Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

What is the size of your Inbox folder?

When I use right click on properties and then work down to the menu that has repair, our personal Yahoo account that has no problems shows no location for the Inbox files.

Every folder should have a 'Location' information regardless.

I have tried cutting and pasting from one of our other Yahoo accounts (planning to change the number between Yahoo and .com), but it won't paste.

Stop trying arbitrary things, it may just make matters worse.

We have Thunderbird with the same five accounts on two other computers, so would it make sense to copy the inbox files from another one for that one account?

What is the account type - POP or IMAP? I'd not copy anything at this time until you fixed the problem with backing up the Inbox file from the troubled computer.

The three Yahoo accounts are all Pop3. (The non-Yahoo accounts are IMAP.)

There really is no location information. I can believe there should be, but it is blank.

One of the early steps was turning off anti-virus monitoring of e-mails. We also tried Safe Mode on early steps with same results.

Since Spinrite isn't free and I have no experience with it, and since we have a GeekSquad subscription, we'll let them check the disk along with a general checkout. (We live in Central Africa but are in the US for a few months, so it's a good chance to check everything out for incipient problems.)

I have backed up the whole Thunderbird profile manually, not using MozBackup since it gave funny results when I earlier backed up my own computer.