I’ve been playing around with Shiira this weekend, a Mac browser with a few interesting features. As an alternative to tabs, you can have what looks like a drawer of screen previews at the bottom of the page. You can also have a floating, black, partially transparent bookmarks window, which is pretty cool on a widescreen setup.
I’m not sure any browser is close to unseating Safari as my primary, but Shiira may fit right in with Firefox as one of my “mood browsers.”