Just installed Thunderbird on a new Windows 10 PC and exported the Thunderbird Profile from my old Windows 7 PC to a "Thunderbird_profile_backup.zip" file, copying that file to a USB stick for transfer to the new PC. But when Importing that zip file into Thunderbird on the new PC, and only selecting "Mail" to Import, only the top Inbox, Sent, Trash, & Drafts mail folders (with files) transferred - no subfolders with their files. I couldn't find any option to include subfolders during the Export or Import procedure. What could be the problem? (Zip file was less than 2GB)