Well, you can't add bookmarks via script in any browser other than IE. I just found that out today
The best you can do is change the script to check if it's IE, then add to favorites, if it's opera, alert the user to press CTRL+T, otherwise, alert the user to press CTRL+D.
This forum thread had some good ideas:
http://www.webdeveloper.com/forum/showthread.php?t=81635