gistfile1.mkd

in your html

<link rel="stylesheet"  src="site.less?browser=ie" />

site.less

.opacity(@alpha: 100) {
  .opacity(@browser, @alpha);
}
.opacity(ie, @alpha) {
  filter: alpha(opacity=@alpha);
}
.opacity(ff, @alpha) {
  opacity: @alpha / 100;
}

Now to use that elsewhere in your less file:

#header {
  .opacity(50);
}