I don't understand ! You want to run a specific application and nothing else, as you said in your first post. If that is what you want, the before mentioned command can get it running. Why do you need window manager ?
I just tried this myself and firefox launches but, without any borders and title bar etc as the WM is not running. But, if all I wanted was just the firefox running, I wouldn't need a border, status bar...... !
If the only program you can launch is xterm and starting any other program fails, then, you should come back and describe what exactly was the error message. Probably, the error has to do with you configuration. Post the error message here, if you really want to make it work.