I can only assume you are using POP rather than IMAP - if so haven't
you used the switch leave files on server. That would have made life
easy. The imported folders should be in a directory called outlook.sbd
in a funny named directory like pr4qpneu.default. Don't know where
your .pst comes from.

In TB imported files are at
thunderbird/profiles/pr4qpneu.default/mail/mail folders/local folders

kb.mozillazine.org/Importing_and_exporting_your_mail

alternatively use the ImportExportTools and you can export as mbox or html.

Failing that why not stay with TB [ver 3 just out and thus more up to
date than outlook] - i've just spent several days looking at different
mail clients and come back to TB.