For anyone wanting to go to Firefox 4, but not liking the UI overhaul, I
recommend the following extensions:
Firefox 4 UI Fixer
<https://addons.mozilla.org/en-US/firefox/addon/firefox-4-ui-fixer/> -
Allows movement of the menu bar (I can't move into the Customize window
to get rid of it entirely, but I moved it to my bookmarks toolbar, which
I always keep hidden), adds the Firefox menu as a button (so you can
have the standard title window /and/ the Fx menu), and makes status bar
icons movable.
Status-4-Evar
<https://addons.mozilla.org/en-US/firefox/addon/status-4-evar/> -
provides a place for items that use the old status bar and don't have
buttons.
Toolbar Buttons
<https://addons.mozilla.org/en-US/firefox/addon/toolbar-buttons/> - Tons
of buttons that let you have common functions available in a toolbar or
the add-on bar.

Firefox 4 still has UI issues (font rendering being the biggest), but
the speed increase is well worth the switch.