[mx]

we used this little piece of javascript to detect Firefox on a Mac. Pretty basic stuff and easy adapt to suit your requirements.

function detectFirefoxMac(){
	var userAgent = new String(navigator.userAgent);
	var platform = navigator.platform;

	if (userAgent.search("Firefox")>-1 && platform.search("Mac")>-1) {
		return true;
	} else {
		return false;
	}
}

I believe it's possible to spoof your user-agent and platform though, just so you know.