For old Thunderbird MBOX files, I’d first keep a backup of the original files and try opening/importing them directly in Thunderbird. If that becomes difficult, especially with multiple or large MBOX files, SysTools MBOX Converter can be another prac...
I agree that better Apple Mail and MSG support in Thunderbird would make migration much easier, especially on macOS. If Thunderbird’s built-in options don’t work, another workaround is to convert the mail files into a more compatible format first. I’...
If it’s just one Office 365 account, you can first add the account to Thunderbird using IMAP and let the messages sync. For a larger mailbox or when you want a more controlled migration, exporting the Office 365 emails to MBOX and then importing them...
