When I try to import a .csv contacts file from Windows Live Mail, the TB menu of destination fields varies with each source field, and in most cases none of the TB fields offered are suitable.
Work-arounds suggested by the TB community involve using Excel to match the .csv fields to the TB fields.
It is suggested that all TB fields be shown for all source fields, so any TB field can be selected for each source field. This is done in Outlook, so should not be difficult to implement.
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As far as I can see, that is how it is done.
Unfortunately, WLM has a mass of fields it exports to CSV that have no Thunderbird equivalent field. It is not as is you can assign two CSV fields to the same Thunderbird field.
BTW I have seen a lot of recommendation to use a spreadsheet. No one I have seen recommends Excel, as it further complicates an unnecessarily complex issue by trying to import Unicode text.