SImple CSS hack for Firefox 4.

Using :-moz-any as a CSS hack for Firefox 4. Hacks are evil, though. Better use feature detection.

HTML

<div>I will only be RED in browsers supporting :-moz-any (Gecko 2 or newer).</div>

CSS

div {
    color: red;
}

/* Override the above rule in Firefox 4+ / Gecko 2.0+: */
:-moz-any(div) {
    color: red;
}