I haven't used xfce for more than a year now however I think in its settings window there is a section dedicated to the session configuration. And in there somewhere you can add apps which will start along with xfce.
If you are starting xfce for the first time I recommend starting it using xinit. You should have GTK_MODULES=gail:atk-bridge in your ~/.xinitrc , you should start xfce4-session from within the same file, then on the first launch press alt+F2, type in orca. When orca comes up talking go to the poxfce4-popupmenu by pressing alt+F1, explore your choices, go to the settings and activate Accessibility in there. If you would like to have orca started automatically with xfce4-session also try to find the settings I was talking earlier. After all this is done install older or patched version of lightdm-gtk-greeter and after that you should be good to go and enjoy your xfce and lightdm experience.
