creating an 'add to bookmarks button' that works accorss different browsers

Hello, I am trying to create a "add to bookmarks" button that will work accross all of the popular browsers. Its easy enough for IE but I am not having any luck with firefox/netscape or opera. Although I would like it to work accross all the different browsers firefox/netscape and IE are the main ones I am conserned about. I have tried:

and

In the head:
<script type="text/javascript">
var bookmarkurl="http://www.yoursitehere.com"
var bookmarktitle="Your Site Title"
function addbookmark(){
if (document.all)
window.external.addFavorite(bookmarkurl,bookmarktitle)
}
</script>

And the body:
add this site to your favourites

Can anyone advise me on how I could go about this?

Thanks in advance