File>offline>download/sync downloads the messages from all folders that were authorized for offline access as defined at account settings>synchronization&storage and the Advanced button. This is done for all IMAP accounts in the profile.

OK so this part is fine, they two accounts are downloaded locally but I don't see where those two specific downloads are saved within the /profiles/ folder structure.

Mike

They are downloaded within the existing folders, not separate folders. The process is to ensure that you don't just have the headers but all of the content.

I will show my ignorance and ask which of those 30+ folders and what sort of file(s) am I looking for?

Is it the moral equivalent of the Outlook PST file?

Mike

If you're looking at the profile from with Windows File Explorer, you will see files with the same name as the account folders, such as input, sent, draft, etc. Each file is in two parts, such as inbox, inbox.msf. The msf file can be ignore for backup purposes as it is the index, which thunderbird would rebuild automatically. The file itself, such as Inbox, is technically in mbox format. An easy way to reload into a new profile would be to install the addon 'importexporttools NG'. then, Highlight 'Local Folders' and rightclick and one of the displayed menu options will be importexport and it offers several import options for mbox files, letting you import individual or multiple in one click.