Where does Thunderbird, on OSX, store its offline folders?
I'd say you are looking at the right place.

I've configured Thunderbird to use IMAP to sync with Gmail. In that scenario, my sent emails are located at

/Users/&lt;username&gt;/Library/Thunderbird/Profiles/&lt;randomcharacters&gt;.default/ImapMail/imap.googlemail.com/\[Gmail\].sbd/Sent\ Mail


This SuperUser answer has a screenshot which shows how to look up the location for a particular account: