Unfotunately OpenText's Export Tool does not support this quite old Version of FirstClass. Anyway I can't find it on their homepage and I'm not absolutely sure whether it is available for free use, though the employee of opentext said so...
I can get all mail via offlineimap from FirstClass 5, but I had to unsend all Sent mail to get them and they don't seem to have IMAP-Readable From and Subject fields, so all Sent mail appear without Subject or sender (me). | 
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