"When you start up Thunderbird for the very first time in
Windows, you are greeted with a Import Wizard. You can select
Outlook and import the existing server settings, mail, and address
book. However, if you are moving to Thunderbird in Linux, importing
won't be so easy. Nevertheless, we'll tackle how to manually export
and import your data. Plus we'll see how to grab your other
existing data; your calendar, tasks, and notes.
"Tip: If you are staying within the Windows platform but
changing PCs, you may consider first installing Thunderbird on the
machine that Outlook is installed on. This way you can use the
automated import wizard to load the data. Then you could move the
Thunderbird profile to the desired computer. Just remember you
still must manually bring in other data, if you want your old
calendar, tasks, and notes. Since you could encounter errors when
moving the profile to a different OS, you may just want to go the
manual route for all data when migrating to Linux."