Import Thunderbird mail to Office 365
To import Thunderbird mail to Office 365, I recommend you use the most reliable tool, which is the eSoftTools Thunderbird to Outlook Converter Tool. It can export single as well as bulk Thunderbird mailboxes to PST files. It has 2 ways to convert Thunderbird to PST files: 1. Autoload configured Thunderbird profile position 2. Manually choose the Thunderbird file to migrate.It can move the Thunderbird MBOX folder in many forms, like PST, EML, EMLX, MSG, Yahoo, Office 365, GMAIL, and HTML files.
For more info- https://www.esofttools.com/blog/how-to-import-thunderbird-mail-into-office-365/

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gloria dsouza commented
For all users, SysInspire MBOX converter software for Windows is a wonderful Conversion utility. With Simplified Convert MBOX to PST, you can convert a huge number of MBOX files to Outlook PST files. This shareware quickly and easily converts mbox files from Apple Mail, Sea Monkey, Thunderbird, and other email clients to PST files. This PST file is compatible with all Outlook versions.
Read More:- https://www.sysinspire.com/mboxconverter/
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Peter Boyce commented
SysInspire Thunderbird to Outlook converter software is the best MBOX converter programme for converting Thunderbird MBOX emails to PST files. It has two options for defining the conversion area of Thunderbird emails: - Automatically load the Thunderbird Profile section; manually select the Thunderbird Profile. The converted PST file from Thunderbird is in Unicode format, making it easy to use in MS Outlook 2019, 2016, 2013, 2010, 2007, and 2003.
Read More:- https://www.sysinspire.com/mboxconverter/