The mail connects cleanly in the current era, the rest connects with add-ons, and one tenant switch decides whether any of it connects at all. In that order:

Mail: modern Thunderbird speaks OAuth2, the sign in 365 requires, so account setup with your address autodiscovers outlook.office365.com for IMAP and smtp.office365.com for sending, authenticating through the Microsoft login page inside Thunderbird, MFA included. Where autodiscovery stumbles, manual settings with authentication method OAuth2 on both servers completes it. The full mailbox syncs down with folders intact, ordinary IMAP life thereafter.

The tenant switch, checked when sign in fails mysteriously: organizations can disable IMAP per mailbox or tenant wide, and IMAP off means Thunderbird's native path is walled regardless of settings. The ask to your admin is IMAP with OAuth2 enabled for your mailbox, a modern authenticated arrangement many security teams accept where they rejected the old password IMAP. Refusal moves you to the paid add-on lane: Owl for Exchange connects Thunderbird over Exchange Web Services instead of IMAP, tenant IMAP policy bypassed legitimately since EWS is the protocol Outlook features ride.

Calendar and contacts, the add-on layer either way: Thunderbird's calendar adds the 365 calendar through TbSync with the Exchange ActiveSync provider, the same pairing syncing the contacts as covered in its sibling thread linked alongside or through Owl's coverage when you went that lane. Expect the add-on layer to trail Microsoft's changes occasionally, the standing cost of the alternative client life, mail itself staying the sturdy native piece throughout.