Let us start at the beginning.

1. Is the account IMAP or POP. My feelings and advice will be very different for the different protocols. But I am assuming POP for the time being. 2. Seriously consider upgrading your operating system. No testing is done on windows 7 as far as I know and it is so far out of support that antivirus products are starting to really cause issues. Add to that this is the last version of Thunderbird that will support windows 7. 3. NSTMP files and hence folders in Thunderbird are a result of failed attempts to compact the contents of the folder. That is an essential function around keeping Thunderbird running. What the NSTMP file is, is a copy of the folder with all the previously deleted mails removed that was destined to replace the existing folder when it is complete. When the compact has built the new storage file it deletes the existing one and renames the nstmp one.

Now we get to where you are. Your antivirus is probably out of control and you need an exclusion in it somewhere to stop it playing in the Thunderbird profile directories. While it is possible to get NSTMP files simply by pulling out the power plug, they are generally only caused by antivirus blocking the process with it's own inane meddling, resulting in a compact failure.

First, select the folder with the missing mails and right click and select properties. Then the repair button. In almost all cases that will return the list of email to the folder. The generation of the index file that populates the user interface was messed up usually by a third party application and a repair rebuilds it.

Note that in most cases the NSTMP file is a complete copy of the folder at the time of compaction, but it is not guaranteed.

See https://support.mozilla.org/en-US/kb/compacting-folders and especially https://support.mozilla.org/en-US/kb/compacting-folders-potential-complications

Tried all those things but no luck. Thunderbird INBOX shows 0 emails. I suspect we're out of luck. By the way Thunderbird is running IMAP, not POP. As far as Windows 7 is concerned, my wife's machines is not capable of running anything else at the moment. I'm still stumped as to what happened.

The same dialog as has the repair reports the size on disk. What is it?

The same dialog as has the repair reports the size on disk. What is it?

The size of what the nstmp files?

I have since discovered they are what Thunderbird creates if the folder compacting routine is interrupted, so they probably won't be of any use to me. I have tried Windows restore routine which claims to be able to put back the IMAP directory as it was in late January, but that fails as well because as soon as Thunderbird is opened, it polls the IMAP server and then deletes all the old messages which have just been recreated. In a nutshell, there appears to be no way any of the email can be resurrected.

No the inbox.

DO the emails appear when you use your providers web mail to look at the email in a web browser?

No they don't. They wouldn't because with IMAP the servers deletes messages after they've been delivered. When I restored the entire IMAP directory using Windows facilities, messages from the date of the restoration appeared momentarily until Thunderbird synced with the server. Then POOF, they were gone.