Foxmarks now has a choice of ftp / http / https as choices nowadays - but is still a security risk as it is outside your network (for me this is acceptable - the passwords stay on my machine).
(tried https and it works)
I see from their FAQ you could run this on your own server
http://www.foxcloud.com/wiki/Foxmark..._own_server.3F but I've not tried it
Importantly, for me, it works well across Linux and Windows and doesn't require that the computers are on at the same time.
if you get an XML import / export / synchronisation working it would good to hear about it (there is a similar project I want to do!)
so ignore the comments above!