What fundamental new is for new UI comparing with 3.6? I use Firefox for some way, only for firebug mostly. I think its just evolution of person, now I use what is most serves my needs.
The main advantage over Chrome is that it doesn't freeze up when it's loading a new tab.
Here is a screenshot comparing FF 3.5 to 4: http://cache.gawker.com/assets/images/4/2009/12/500x_firefox...
(3.5 is right)
I agree it's a problem in Thunderbird, which has a menu by default.
I think that if you try to tweak Firefox 4+ to look like 3.6 you're going to run into suboptimalities. It evolved. I think that if you get used to the changes, they're actually changes for the better (new menu giving more vertical room, tabs-on-top, etc).
It's not exactly like 3.5, but it's not transparent anymore.