Thunderbird data cannot be directly imported to Office 365 because its mail format does not compatible with it. Now users have to anyone method: Manual or Third-party tool.
For manual conversion read here: Manually migrate Thunderbird to Office 365
With the third-party tool, the user has various options to load and export Thunderbird mailbox to Office 365, PST, and Live Exchange format. The software preserves folder hierarchy, rich text formatting, inline images, attachments, and other mail properties in an Office 365 account. Users can install the software on any Windows machine and perform hassle-free conversion from Thunderbird to Office 365.