I would report this to the mono bugzilla. I have not analyzed the markup but I have not noticed any problems with mojoportal skins under mono not rendering correctly in the browser. I think its possible issues like this are caused by differences in browser definition files on the platforms. I'm not sure debian gets the same browser definition files that opensuse gets. So if there are differences it can affect whether the control adapters work to render the menu.
Or it could be worked around possibly by adding css to handle either rendering the same.
Best,
Joe