Perhaps ask Microsoft how to import your data. I have no idea what tools they provide (probably none) but seriously there is no formalized task format, so I doubt you can do anything with them. Calendars are generally managed using iCAL files, and Thunderbird exports them. What Microsoft does with them is again something you need to address with them. Thunderbird will export your contacts as a CSV file (one per address book) and I know outlook used to import that. I have no idea what Office 365 does. I assume it also supports them I see not reason for an MBOX to PST tool however as Thunderbird only stores mail in an MBOX formatted file and you say you have already moved that. But all of that migration stuff depends on where and how your data exists not. My contacts in Thunderbird are synced with google for instance, so I would not be importing them from Thunderbird. Likewise my calendars are on caldav server on the internet (Google Yahoo and GMX). Microsoft however uses a proprietary calendar solution, so If you have remote calendars you will need to ask Microsoft how you import using their proprietary solution.