How get Stage.height 100% in Mozilla?

Problem: Mozilla doesn’t recognize Stage.height as other browsers do. I like to place a object relative to the height the users like their windows to be. Say I want to place a mc_ 30px above the bottom of the stage. In most browsers this is not a problem.

Like this:
mc_._x = Stage.height-30;

But, in Mozilla, no go. Can anyone help me with this? I’ve searched the web for a couple of hours, but found no solution yet … Also, when I trace the value in Mozilla, I always get “Height:200”, but the Width works fine (!?)