Office 365 is a cloud platform where users can import only PST files manually. While Mozilla Thunderbird stores data in MBOX and Maildir files that the user cannot convert to Office 365 directly.
So when a user wants to transfer a database from Thunderbird to Office 365 mailbox then I would like to suggest you try MailsDaddy Thunderbird to Office 365 Migration tool.
This application allows users to import data into Office 365 and Outlook mailboxes without any hassle. Plus it has two mailbox options: primary and in-place archive. The free demo version of the tool converts 20 emails from Thunderbird to Office 365 account.
Read more: Thunderbird to Office 365