I've found myself using Firefox 3 more and more as of late, though there is one feature of Safari that I miss dearly. Picture it: you have a bunch of tabs loaded up in one window of your browser. For some reason you decide that you would really like to see one tab in its very own window. In Safari all you have to do is click on that tab and drag. As you can see to the right, a little picture of the website you're viewing appears and you can drop that picture anywhere on your screen. Once you drop it, the picture expands into a Safari window displaying that website.
This cool feature is a little tough to explain in words, so read on for a brief video demonstration.
Please note that I'm using Leopard and Safari 3.