browser.tabs.onTop Boolean (Firefox 4.0 and above)

Determines position of the Tabs Bar relative to the Location Bar. True (default): Tabs Bar above Location Bar False: Restores old behavior of Tabs Bar below Location Bar Note: bug 755593 will soon remove tabs-on-bottom mode. Once it is RESOLVED FIXED, this pref will no longer be effective.

I think this can do what you want,

Classic Theme Restorer (Customize Australis) {web link} Restore squared tabs, appmenu, add-ons bar, small button view and more on Australis UI (Windows/MacOSX/Linux + Fx 29+).

This will restore many of the things the last upgrades took away.

http://forums.mozillazine.org/viewtopic.php?f=48&t=2827985 Classic Theme Restorer - Discussion Thread (v2)

edited email address out from public and spambots. it is listed at a.m.o page Classic Theme Restorer - Support E-mail

See also:

• Tabs On Bottom: https://addons.mozilla.org/firefox/addon/tabs-on-bottom/

You can look at the userChrome.css solution I posted in this thread to move the tabs to the lower position just above the browsing area without using an extension.

• [/questions/1049464] Bookmarks toolbar POSITION resets to the very top after closing firefox