You already know that border-radius is implemented in different ways in different rendering engines using different (mostly non-standard) property names. The trick is to use them all in for the same element or class. If should work for Firefox/Seemonkey, Safari/Chrome, Opera and IE9.
For example, see the direction and samples here:
http://www.css3.info/preview/rounded-border/[
^],
http://www.the-art-of-web.com/css/border-radius/[
^].
If you need some more or in doubt, you can find tons of other samples if you Google.
Good luck,
—SA