The UI feature I will miss most is tabs-on-bottom. I have an address bar, a bookmarks bar, and a row of tabs. Quickly scanning tab titles and switching between them with the mouse is more important than knowing the current page's URL. So the tabs should be closer to "the action", the web page my eyes and mouse are interacting with (Fitt's Law).

The "Tabs on Top" menu item has been hidden since Firefox 15. The about:config pref "browser.tabs.onTop" is the only way to toggle this feature.