I actually still run 3.6 since I like the UI on it better. The speed of the browser means very little to me, I'd rather have the familiar UI. If you liked FF 3.6, why not stick with it?
And FF 7 (which I've been using for about 5 months now) is not even noticeably faster than 3.6 on my usage pattern. In fact it seems to spend more time with an unresponsive UI than 3.6.