There are quite a few performance improvements going into Firefox 4, with a strong focus on startup time. See http://autonome.wordpress.com/tag/startup/ for detailed tracking of Firefox startup improvements.
I wish there were a version with less chrome and more web real estate - I've thinned my current version down as much as I can really (Cool Menu 2 FTW) but the tab height is about twice the font height and the menubar (in which I have my location and search bars) is twice the icon height. Why waste all this space - I understand negative space but really this seems to be too much form not enough function, browser chrome should be as slim as possible but still usable IMO; if I use a small font it should be smaller.
These designs are not fully implemented in current nightly builds, and are subject to change. Also, here are the Windows and Linux mockups:
https://wiki.mozilla.org/Firefox/4.0_Windows_Theme_Mockups
https://wiki.mozilla.org/Firefox/4.0_Linux_Theme_Mockups
There are quite a few performance improvements going into Firefox 4, with a strong focus on startup time. See http://autonome.wordpress.com/tag/startup/ for detailed tracking of Firefox startup improvements.