Enable Resize Grip in X Windows

I customized my X Windows to run two applications only. One is a customized program and the other is Mozilla Firefox. Windows Manager is Motif Windows Manager.

Since the default Firefox size is too small, I need to enable Firefox sizing. However, I do not want to use mwm*clientDecoration = resizeh because it does not only show Resize Handler but also the whole border. I need border-less screen.

Would you advise me how I can show the resize grip on Firefox at the bottom right corner only, please?