In the case you describe, moving the Firefox window to the left or right should be sufficient. But in the screenshot it looks like Firefox is inside a vertically split container which is in turn inside a horizontally split container (H[V[Navigator]]), in which case you might need to move the window right or left twice: The first move puts it outside the vertically split container the second one then outside the horizontally split one.
Explanation
Generally, to move a window outside of a container, you just have to move it over the border of the container. If it was the last window in the container, the container is removed (with the exception of the workspace container, which only goes when the workspace itself goes).
If there is only one window in a container, moving it in any direction will move it out of the container (as the window touches all container borders) and thus close the container. But if you want to keep the general placement of the window, you have to move in a direction that keeps it inside the outer container in which you want to place the window (in your case the tabbed container). For tabbed and horizontally split container move it to the right or left, for stacked or vertically split container move it up or down.
In the case of multiple nested containers with only a single window it is sufficient to move the window in the direction of the outer most container (the one in which you want to put the window) until the window is directly inside the desired container. There is no need to move the window through each single layer and switch between up or down and left or right movements, as moving the window left out of multiple nested vertically split (or stacked) containers, will move it out of all of them (the same thing for moving up or down out of nested horizontally split or tabbed containers). So if the order of the two split container around were reversed - V[H[Navigator]] - it would be sufficient to move it to the left (or right) once as the movement out of the horizontally split container will also take it out of the vertically split container.
