Since I.E.'s opacity on text looks ugly (http://jszen.blogspot.com/2005/04/ie-bold-text-opacity-problem.html - could be fixed by adding a background but that's a pain), I suggest that when z.now == 1, you just get rid of the alpha opacity filter instead of setting it to z.now*100 = 100 which will render ugly.

So I propose that you change the code within prop == 'opacity' to something like

if (jQuery.browser.mozilla && z.now == 1) z.now = 0.9999;
if (window.ActiveXObject) {
	if (z.now == 1)
		y.filter = ""; //no need for alpha filter if opacity is full
	else
		y.filter = "alpha(opacity=" + z.now*100 + ")";
} else {
	y.opacity = z.now;
}