Yep, this is something that's been bugging me for a few years now. As we all move to ever-increasing screen sizes, you don't really want/need the main browser content to be full screen, so there's all that screen retail space available to do something a bit different. I like what they've done with the pop-up previews and could see it working on a PC/Mac version of Firefox.
Seems to me that browser developers aren't really moving with our changing needs - I'd like to see more innovating in web browsers, nothing out there at the moment seems to offer anything to use that extra horizontal space for useful tasks. Safari's "Top Sites" feature was a step at doing something different, but it was implemented as a bit of an add-on afterthought, rather than integrating it into the interface. I thought 10.7 Lion's version of Safari would have offered something like this, drawing on iOS's influences, but it appears to be the same, drab, old interface - bit of a wasted opportunity really.
