NSTMP files are created as a part of the compacting process and only appear if the process was interrupted before it was complete.

This could have been because you closed the program, but a more common cause is an anti virus product that just bogs the compact down until it stalls. To try and identify and workaround this issue, use the compact command on the File menu (Alt+F then press F again) do this as soon as you have completed receiving email after you stat Thunderbird and don't close it for perhaps half and hour.

I do not shut Thunderbird down. It stays open all day long. Folder compacting used to work fine, but over the last week or so the nstmp folders started showing up and not going away (I see it appear in the morning and its still there in the afternoon).

Would folders that contain very large amounts of data/emails (several GB) cause it to stop? Is there a limit to the size that compact can work on?

I am going to start trying to keep track of which specific folders are causing this issue.

Thanks

Size should not matter at all. The 4Gb limit on folders was lifted some years ago. While there might still be bugs hidden away that come out to play in folders over 4Gb, they would be quite rare. I have not seen anyone in support mention these folders in a couple of years that I recall.

I would guess it is an issue with file contention. The two common causes of tht these days are streaming backups/cloud synchronization and anti virus programs assisting but causing failure. Hence the common recommendation that the programs be prevented for on access scanning iun the profile folders through an exception in the product.

Give this info a read: http://kb.mozillazine.org/Nstmp_folders

I use AVAST antivirus but everything is turned OFF and I just manually run a scan every now and then. And no streaming backup, as I dont use cloud... everything is on my computer.

Are you talking about a pop or imap mail account ?

I was thinking it may be more to do with this: Thunderbird creates the "nstmp" file in the same directory as the folder it's compacting, and then renames it. This means it's possible that you might not be able to compact a large folder because you don't have enough free disk space. If this occurs, try manually compacting some smaller folders one by one to free up enough space.

To find out if compacting is actually working ok on the large folder.... Suggestion - for what you call a folder with 'large amounts of data/emails (several GB) ', create another folder and move approx half of the emails from large folder into it. Then perform a manual compact on the original larger folder. If you are using imap account folders then suggest you do this in offline mode to stop any synchronisation whilst performing this as it means moving alot of emails.