Not sure when Mozilla removed the ‘Tabs on Top’ option from the Toolbars sub-menu (View > Toolbars) but it is gone for sure in new installs of Firefox 14/15. You can however, still turn off the Tabs on Top option (but the UI is looks a bit odd) via about:config setting:
- Open a new tab. In the address bar type about:config and press enter
- (If applicable) Click on the I’ll be careful, I promise… button
- In the filter box type browser.tabs.onTop
- Double click on the entry below to change the setting to false
- Close the tab
Update: After some testing it appears the ‘Tabs on Top’ menu option is missing if it is a fresh install of Firefox. On the laptop I put a new install of Firefox 14 on about 3-weeks ago and then updated this week to Firefox 15. On the Ultimate machine I just did a fresh install of Firefox 15 and ran it with a new profile and the option is not there. Also noticed it missing from the latest 64-Bit Windows Nightly which I did fresh install of within the past couple months.