You can switch between Tabbed and Normal mode on the container. Just right click the container and either check or uncheck the "Tabbed" box (in tabbed mode) or click the "Set as tabbed container" (if you are in Normal mode).
Tabbed
In the tabbed configuration the windows will show as tabs at the top of the container. When a buddy sends you a message, their tab will flash. You can just click that tab to switch to that chat window.
Normal
In Normal container mode, the container will act more like your desktop. The windows will remain re-sizable, the taskbar will appear at the bottom of the container. You can move the windows around and resize them in the container if you choose.