To be honest I'm ok with the new interface, considering that I'd already tweaked FF 3.x to look almost exactly like this even before they announced the new design; I want as much space possible for my content, and the lack of menus and toolbars (which I never use anyway) is offset by mouse gestures and keyboard shortcuts.
Now, if you're on Windows 7 or Windows Vista SP2 you can speed up Firefox 4 some more by enabling Direct2D and DirectWrite hardware acceleration which will use the GPU for things like native text rendering and the rest.
To do this just go to about:config
Type render in the filter box
Look for gfx.font_rendering.directwrite.enabled - right click and select toggle to turn it to true
Look for mozilla.widget.render-mode - double click and set it to 6
If you don't have mozilla.widget.render-mode just right-click on empty space and select "New -> Integer" and set its name to mozilla.widget.render-mode.
f you want to disable the hardware acceleration, set "gfx.font_rendering.directwrite.enabled" to false and "mozilla.widget.render-mode" to 0.
This is how FF 3.x used to look for me, so FF4 isn't really that big a change.