Firefox 3

Handling both Firefox 2 and 3 in your CSS files sometimes turns into a tough job.

This nifty little hack does the job:

/* Firefox 2 stuff */
.foo {
  background-color: #fff;
}
/* Firefox 3 stuff */
html>/**/body .foo {
  background-color: #000;
}

And guess what? This block of CSS validates fine on W3.

Found on: http://snipplr.com/view/7043/firefox-3-css-hack/