What I posted will round them. You may want 10px instead. And, you may want to get rid of that border on top (background image) and maybe make it taller (padding-top and top), but I posted the code to make them round.
Would like this on mine but added the code and it does not appear to do anything ?
If your in IE or a browser that does not parse moz-css effects you will not see it.
To achieve this in every browser you must do some tinkering and make the tabs all the same size then have a new navtab background made. Other ways to go about it but plainly put, I've done a few styles recently with a customized navbar using images and modifying the css.