This has been annoying me for almost 2 weeks now. Entry #7 of this fixes the problem for me.

In short:
in /usr/share/xfce4/helpers/firefox.desktop, replace the last two lines with


X-XFCE-Commands=%B -new-window "about:blank";%B;
X-XFCE-CommandsWithParameter=%B -new-tab "%s";%B %s;