It looks like Acid2's nose has pimples at the left corner and bottom corner. I see this problem on Mac but not on Windows.
bug 343583 is somewhat related, for what it's worth.
This is a duplicate of 343583. The rendering of the nose isn't really defined in any spec, so it's correct. The final version of Firefox 3 will render the nose antialiased, so it will be different from both the reference and the current version. (Somebody, please, dupe this to bug 343583!)
Using anti-aliasing for borders is bug 328241, I think. But it's not clear to me from skimming that bug that it will be fixed for Firefox 3.