Crossbrowser opacity is one of those things everyone's just after, especially after the strong rise of CSS3. Here's the beautiful code that should allow you to obtain Crossbrowser compatibility for opacity:

.crossbrowseropacity
{
  background-color:#000;
  opacity: .75; /* W3C Standard: FF > 1.5, Opera, Safari */
  -khtml-opacity: .75; /* Safari 1.x */
  -moz-opacity: .75; /* FireFox<1.6 */
  filter: Alpha(opacity=75); /* IE<8 */
  -ms-filter: "alpha(opacity=75)"; /* IE8 */
}
.crossbrowseropacity *
{
/* CLEAR CHILDREN */
  background-color:#000;
  opacity: 1; /* W3C Standard: FF > 1.5, Opera, Safari */
  -khtml-opacity: 1; /* Safari 1.x */
  -moz-opacity: 1; /* FireFox<1.6 */
  filter: Alpha(opacity=100); /* IE<8 */
  -ms-filter: "alpha(opacity=100)"; /* IE8 */
}