As you notice, FX use tabs, so you can't put tabs in front of an other like any other real window.
I can't test FX here, sorry.
I tried with Chrome to see if it's possible, and it's not. \o/
In fact the tab which is not active doesn't exist as a window, so you can't activate it...
If fact, in most cases if the window is not visible in the Task Manager of Windows you can't activate it (excepte if you know the title of a hidden window, but it's not the case here).
FX and Chrome use a sort of container to display the tabs, and the thing which is active is the container of the active tabs, so you understand you can't activate something which doesn't exist.
