I tried to configure freemind to use a running mozilla/firefox instance instead of starting a new one:

  firefox -remote "openurl({0},new-tab)"

This didn't work since the "{0}" inside the argument was not replaced by the actual url. It seems that freemind only replaces a "{0}" if it stands for itself.

There is a simple workaround, though, see http://gnome-hacks.jodrell.net/hacks.html?id=48