When I tried to select a file named "-b" xffm crashed. it also crashes if the object is a directory
What version of xffm? With CVS HEAD (4.1.2) I cannot reproduce the problem. Could you post a gdb traceback of the crash please. If you are using CVS HEAD you will have a core file at $HOME/.xfce4/xffm/xffm.core. Run with $gdb -c xffm.core xffm to produce traceback.
Seems like it might have been a problem with the local xffm build. Reopen bug with gdb traceback if problem appears again.