My browser of choice is Firefox Nightly which is currently V20.0a1.
I like to see the development process where the developers play around with various layouts and techniques, such as how they handle downloads and the general layout.
Lately they made a change which I found a little frustrating – they moved the tabs above the toolbars and removed the option to change this from the menu. I am used to the old “tabs on bottom” layout so had to find a way around this.
The solution is, of course, simple….
In about:config just toggle the entry – browser.tabs.onTop to read “false”.