Tiny Menu Firefox Plugin. This plugin will hide all the normal menus and make them available under 'Menu'. (In my case I've configured it to keep File and Tools instead of hiding them). Then customise your toolbars (click View/Toolbars/Customize). It's a little hard to figure out what to do there but you can drag things around. I've put my bookmarks toolbar up onto my menu toolbar and put the menus over on the right. Then hidden the original bookmarks toolbar. It's pretty nice and I get a bit more screen realestate. The extra bling is a Firefox persona of course (which requires Firefox 3.6).
I haven't done this but you can do a lot more by tweaking userChrome.css in your firefox profile directory. See http://www.mozilla.org/unix/customizing.html or google for more examples.