This is not something that I have much experience with but doing a quick search I found this code:
filter:alpha(opacity=50);
-moz-opacity:0.5;
-khtml-opacity: 0.5;
opacity: 0.5;
I got this from the following page:
css-transparency-settings-for-all-broswers/
I think you would want to apply this to the :a class for the links.