There is no such thing as an "Active URL". Every browser window can only navigate to one URL, and every window is the "Active URL".
What you're referring to is which window has the input focus. The ShellWindow class does not track that.
You can use the GetForegroundWindow
] function to get the handle to the window that has the input focus. You can then compare that handle to the window handles you get if you enumerate over the list of processes you get from calling Process.GetProcessesByName()
]and look for "iexplorer.exe".