Press the F10 key or use "Firefox > Customize" to make the Menu Bar and other toolbars visible and remove the check-mark on "Tabs on Top" to place the tab bar in the old position.

Please see

https://addons.mozilla.org/en-US/firefox/addon/firefox-3-theme-for-firefox/

http://support.mozilla.com/en-US/kb/how-do-i-customize-toolbars

Let us know any specific changes you want to make if that doesn't resolve the problem.

When I tempted to restore a known interface I found the same solutions. I obtained a look similar to FF 3.6 (and earlier), but not the same accessibility. I need a real 3.6 interface (now I am loosing opened tabs as they are in uncommon places, far left over my bookmarks sidebar.) Mozilla WANTS to change us in "standard Ie users": you do not really need this, you do not really need that... minimal interface. Mozilla, to be innovative, WANTS we take differents habits: OK, be innovative, but let us keep our habits if we want. I remember, when FF3 was born, the fight on the "Address Bar suggestions". The suggestions were forcefully taken by Bookmarks and History: someone hated this, someone liked. Now in FF v.4, we can customize it in the "Privacy" options tab......... Quite late... but this is the way to follow, Mozilla!

If you don't like the way the Firefox 4.0 User Interface looks, see these two pages.
http://www.computertechtips.net/64/make-firefox-4-look-like-ff-3-6/
http://blog.fligtar.com/2011/01/16/how-to-customize-firefox-4s-ui/