Translating mouse position onto a plane

Please login to see this picture.
(The circle is actual mouse position, the red/green shape is the Mouse Pick X/Y/Z position)

I'm trying to translate mouse coordinates to 3D coordinates by using a plane. As far as I know the only way to do mouse-firefly -interaction is via collisions and Mouse Pick. However, since Mouse Pick is essentially raycasting, the coordinates it returns aren't positioned at the mouse, but rather on some other point that is between the point the Mouse Pick tests and the camera. Changing the Y position of the colliding plane doesn't seem to help; the Mouse Pick test point moves "downwards" accordingly as far as I can see. I can't apply an offset to the values I get, either, because the offset between the actual Mouse Y and Mouse Pick Z position changes based on depth.

So; how would I translate the mouse X/Y to Firefly X/Y/Z more accurately?

Nevermind, my bad; the Engine and Camera objects had a different height. But now that you mention it, do the menubar & title bar affect stuff with Firefly?