Right-click on a toolbar, then click on the "Tabs on Top" entry to de-select it.

There are other changes you can make to make it look more like Firefox 3.6, for details see http://www.computertechtips.net/64/make-firefox-4-look-like-ff-3-6

Right click a blank part of the Tab Bar and then in the menu, click Tabs on top to remove the checkmark. Tabs will subsequently reappear below the location bar.

Also:

• 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/

I also had this problem.. I could get rid of the bookmarks menu that was easy.. I wanted to only remove the individual tabs, but they couldn't be easily removed..

I played with it and found that if you DRAG the tabs to the bookmarks icon to the right of the tabs they would go away.. then if you went into your bookmarks and hovered over those items you could RIGHT CLICK and Delete them..

It should be easier, maybe it is.. I couldn't find it anywhere.. So here it is =)