That is not a folder that Thunderbird creates or uses. Where did you get your new copy of Thunderbird? I hope at the official site and not some random download site like CNET. They like to give you extras with your downloads. Is this an IMAP account? If so your provider might create this folder.

Google it and see what you find.

Hi Thanks for the reply. I think I downloaded it from the official site. Windows Defender scanned it and it was ok. It is very strange. It is a Hotmail account. I ran a search on my System drive and it found a file named Malwarefox.msf in:- c:\Users\xxxxx\AppData\Roaming\Thunderbird\Profiles\xauv8iz8.default\ImapMail\imap-mail.outlook.com\INBOX.sbd. Perhaps someone will know. Thunderbird shows version 52.5.0 (32-bit). I don't know why 32 bit when my system is 64bit.

I don't know why 32 bit when my system is 64bit.

Because Thunderbird is only available as 32-bit app for Windows. This isn't a problem on 64-bit systems.

This is a 3rd party software that is not any part or related to Thunderbird.

https://www.malwarefox.com

You sure you did not download Thunderbird fronm CNET? They are pimping this software on their site. http://download.cnet.com/Malwarefox-Antimalware/3000-2239_4-77478408.html