Would you rather have your tabs back at the bottom of the toolbars rather than the top? This is an easy enough thing to fix with just two clicks of the mouse.
First, you will want to right-click on the Navigation Toolbar. Next, select or un-select the "Tabs on Top" option. When you do so, the tabs will either switch back to the top of the website viewing area or to the top of the window.
So no matter if you are more of a traditionalist (and enjoy your tabs in the old location) or you are ready for new and exciting things (and want to put your tabs on top) there is an option for both in Firefox 4.