Solution: Issue the following command in an xterm (X11) window and then restart X11:
defaults write com.apple.x11 wm_ffm -bool true
The file modified by defaults in this case is ~/Library/Preferences/com.apple.x11.plist.
mkdir /Volumes/mnt (can use any name instead of mnt) mount_afp -i afp://guest@computer.dtm.ciw.edu/ExternalData /Volumes/mnt or mount_afp -i afp://iraf@computer.dtm.ciw.edu/Alycia\ Weinberger\'s\ Public\ Fold /Volumes/mnt
defaults write.org.x.X11 app_to_run xinitrc