I use "filter: Alpha(Opacity=20);" in the CSS code in my website. But when i tried to preview it on mozilla browsers such as firefox and seamonkey, it turns out that the effect is not visible.

are there anyway i can do to make the effect visible?

filter: is a IE only property

other browsers you should use opacity: value; where value is decimal figure

so for 20% opacity you need

.opaque20 {
     /*for other browsers*/
     opacity:0.2;
     /*IE specific */
     filter: alpha(opacity=20);
}

You must give the element an explicit height and/or width as well in IE (or use position: absolute)

Use -moz-opacity: 0.2; for milliza based browsers such as firefox. For an all round broswer support you should include all three codes (shown below).

opacity:.20;
filter: alpha(opacity=20);
-moz-opacity: 0.2;

-Brett