I'd love all Thunderbird address book fields to be supported by the Funambol plugin. At least the fax number and the messenger name are currently not synchronized correctly.
I started with a recovery from my local address book to my new Funambol server, then switched the local address book to an empty one in the settings and did a reverse recovery into this addressbook. When I compared the results, I saw that fax numbers and messenger names have not been copied. Everything else I checked including birthdays have been copied (didn't check in detail), which makes me very happy :-)
See http://lxr.mozilla.org/mozilla1.8/source/mailnews/addrbook/public/nsIAbCard.idl#64 for available Thunderbird address book fields.
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Another point: If I set the fax number with the Web Demo Client, this fax number appears in the home phone number field after synchronization (which makes the home phone number dissappear in the Thunderbird address book).