Problem: After upgrading to Firefox 4 the first "improvement" that you'll notice is possibly the all-too-distracting popup which displays the full target URL whenever you move the mouse pointer over any link. Apparently they got rid of the status bar, but decided that for security's sake the information must still be displayed somewhere and chose a popup. Unfortunately, a popup which keeps appearing and disappearing over the main page display area feels much more intrusive than a fixed status bar area (which you could also hide altogether).
Solution: Install the poorly named add-on Status-4-Evar. It will instantly restore the original status bar without any further configuration.
And by the way, you can also disable "View > Toolbars > Tabs on Top" to restore the original proper location of tabs. To move the Home and Reload buttons back to where they belong left of the address entry field, you can right-click on the little arrow within that field (or almost anywhere else on the toolbar), select "Customize" and then drag the buttons back.
The most comfortable GUIs are the ones we're used to.