It's a means of speeding up page loading I believe.

Here are a few tips to make FF4 look like the previous version.

• Right click a blank part of the tab bar and click "Tabs on top" to remove the checkmark. Tabs will subsequently reappear below the search bar.
  
  
• Repeat the same steps, but click the top one called "Menu bar". This will reinstate the text links at the top.
  
  
• To rearrange the layout, repeat the same steps again and choose "Customize". A panel will open and while that remains onscreen, you can move elements like toolbars, buttons etc around and place them anywhere you wish. You can also drag anything you don't like into that panel and add anything you want by dragging it out of there.
  
  
• To replace the status bar at the foot of the page, install this add-on: https://addons.mozilla.org/en-US/firefox/addon/status-4-evar/
  
  

Doesnt really answer the question there. Shouldnt need addons to have functionality that has always been there.

If it aint broke dont fix it....

I guess you're entitled to your opinion, but IMO Mozilla was smart to get rid of the Statusbar. The Add-on bar is far more configurable than the old Statusbar ever was, it works just like a toolbar at the top of the window.

At the top?? WTF. guess its time to find a new browser.

Yes, the Add-on bar works just like the Toolbars at the top of the browser. Any Toolbar button that is on a top toolbar or in the Customize palette can be placed on the Add-on Bar at the bottom. With the old Statusbar, a developer would have to specifically code their extension to have a Statusbar icon or button - and the user couldn't move that button /icon if they wanted it elsewhere. That limitation doesn't exist with the Add-on Bar.