Whenever you open multiple instances of the same application in Windows 7 it will stack the Taskbar icons one behind the other with no textual indication. So, in order to switch between instances, you need to hover the mouse cursor over the icon to reveal the various thumbnail previews and then select the one you wish to work with.
Here is a typical example for two instances open in Firefox:
I can understand why Microsoft thought that might be a good idea as it helps prevent overcrowding and filling the Taskbar too quickly/easily, but I personally do not like it. I much prefer the Vista system of keeping Taskbar icons clearly separated and identified..
That default behavior in Windows 7 can be changed quickly and easily however via a simple setting.
Right click on the Start orb and select Properties. Click on the Taskbar tab and look for “Taskbar buttons“. From that drop down menu, change the setting from “Always combine, hide labels” to “Combine when taskbar is full“:
Now you have the best of both worlds; icons clearly separated and with textual identification which will revert back to stacking whenever the Taskbar becomes too full: