like its predecessor, the Firefox 5 user interface doesn't quite work on the Mac - it isn't as Apple-y as Safari or as pretty as Chrome, and even applying themes doesn't help much - but it's a lovely thing on Windows.
Whichever version you use, you get the ability to turn open tabs into App Tabs, which keeps key web-based services such as Gmail or Twitter visible without taking up too much space, and you can organise tabs into Tab Groups, which is handy if you're researching something involving multiple topics.
Annoyingly, the only way to save your Tab Groups is by shutting down Firefox; unless you've disabled Firefox's session restore, your groups will be intact when you open the browser again. It'd be much better if you could save groups as you do bookmarks: while you can save a tab group using Firefox's Bookmarks > Bookmark All Tabs option, which only saves the currently selected group. A Save Tab Groups option would be handy here.
As before, Firefox has an address bar and a search box, although if you don't type a URL the address bar takes you to a web search anyway.