I have just noticed something really odd, which I believe might be a small bug in Firefox 9.0.1. Have a look at the screenshot below. It shows Chrome 16 and Firefox 9.0.1 and how they render a fieldset with a border and a box shadow. Also, for your entertainment, a fiddle demonstrating the issue (view in Firefox 9.0.1, obviously). As you can see, Firefox appears to apply the border and the shadow to two different shapes. Border radius is applied to both shapes separately.
It didn’t happen in Firefox 8, so I assume it’s a new bug. [EDIT: OK, according to @ptriek, it DOES happen in FF8. I don’t recall noticing it, however. My bad.]
Any idea what’s going on? Is there a workaround or do I just need to wait until Mozilla is kind enough to notice and fix this?
[EDIT: There is already a bug report at Firefox’s Bugzilla concerning this. Check it here in case you are interested or wish to follow it :).]
You could fix it by adding some absolute + relative positioning:
http://jsfiddle.net/EC5ym/1/