I've been struggling with this issue of Mambo admin menus in Safari since discovering Mambo a few weeks ago. Really liked the CMS, but the Safari problem was a big deal for me. I've been using the Mozilla Camino browser for my Mambo stuff as a workaround.
Just discovered that Version 1.25 of JSCookMenu was recently released, and it appears to fix the Safari menu placement issue. I downloaded it and overwrote my old JSCookMenu.js and it seems to be working great.
I'm uploading JSCookMenu Version 1.25 here. It belongs in [mambo home]/includes/js/.
See the JSCookMenu website for more info:
http://www.cs.ucla.edu/%7Eheng/JSCookMenu/index.html.