You can't make only the background transparent using filter or -moz-opacity--this is the way CSS works.  There is something you can do but it does not work with Internet Explorer because Microsoft is too small a company to implement proper PNG support.  The idea is to use “background-image: url("bg.png");” and make that PNG a transparent image.  You can do something in between that works with IE too using a GIF image, but that's only binary transparency and it doesn't get even close to what you could do with PNG.

About rounded corners, it should be possible but pretty difficult to do.  I'll try to hack a menu theme about this in the next release.