Thunderbird: Free Your Inbox on Nostr: If you can have Outlook save the individual .eml files instead of as a Pst, you can ...

If you can have Outlook save the individual .eml files instead of as a Pst, you can use this Add-on to then import them into Thunderbird: https://addons.thunderbird.net/en-GB/thunderbird/addon/importexporttools-ng/?src=search

This Mozilla Support questions goes into a little more detail - and also has a link to a third party provider that the OP found useful: https://support.mozilla.org/en-US/questions/1154588