This isn't the subject of this ticket, but after doing the migration/import, my Firefox release now complains about profile downgrade, because the MSIX Beta opened it :(
Bug 1728161 Comment 11 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
This isn't the subject of this ticket, but after doing the migration/import, my Firefox release now complains about profile downgrade, because the MSIX Beta opened it :( At least this won't be an issue in the common case, with release Firefox installed and folks trying out release Firefox from the Microsoft Store.