I've spent a bit of time on this now and have just about got something to work, although it's not great.
Firstly, this addon can be used to import MBOX files into Thunderbird:
http://www.nic-nac-project.de/~kaosmos/ ... rt-en.htmlBear in mind you should got to Barca and do folder > Compress All Folders first or you'll find random messages you thought you'd deleted returning.
A couple of issues with this: it doesn't take across the "unread" indicators - all messages come over as unread. Secondly, and more annoyingly, it doesn't appear to take over any attachments.
As far as the calendar is concerned, for some reason I didn't have any luck with an export/import of iCalendar format. So I imported into EC Client then exported again in the same format - this one went into thunderbird ok. (Note: it is possible I did something wrong with the first import so this might not be necessary, but it worked.)
Then I removed all contacts from all groups and exported to LDIF. Unfortunately this is a complete mess and it only imports in a fraction of the information. Not sure if anything can be done or not, so I'm just going to have to bite the bullet and go through each contact in turn adding in their details.
Annoying about the contacts and attachments but doubt there's much that can be done.
