If you're using standard controls then the obvious option is this:
vb.net Code:
  1. TabControl1.SelectedTab.Controls.OfType(Of WebBrowser)().Single().Navigate(theNewUrl)
This assumes that you want to affect the currently selected TabPage and it will always have one and only one WebBrowser control on it.

That said, you may find it more useful to create your own custom controls and build the functionality you need into them, rather than just using the standard controls and fussing every time. Follow the CodeBank link in my signature below and check out my Tabbed Web Browser thread for an example of that.