Possibly related to https://connect.mozilla.org/t5/ideas/should-be-able-to-a-tab-with-other-tabs-on-different-window/idi...
Basically, like in chromium, if I pull the last tab(s) away from the tab-bar of a window the window should go away and reveal what's behind it. This lets me easily merge windows when I no longer need the tabs to be in a separate window.
If I release the tabs outside the tab bar of another window it should just recreate the window that was removed.