If you want to import Thunderbird email to Office 365 file, use the most reliable tool that can convert multiple Thunderbird MBOX files at once without difficulties, which is the eSoftTools Thunderbird to Outlook Converter tool. It has specific features that allow you to quickly import Thunderbird emails to Office 365 files without wasting time, and it can auto-load optimized Thunderbird emails for conversion.
For more info:- Import Thunderbird email to Office 365 file