I have the same problem with Firefox, right mouse menu appear in bottom left of the sreen...

 

I cannot recreate this in your the demo page with Firefox, but it always happens in my implementation...and many others are having this problem as well (according to the forum).

 

I also, in IE 7, the right mouse menu looks really messed up..  it is partially transparent like the z-indexing in wrong for the DHTML.