You can use the keyboard shortcuts:
- Ctrl+Tab to skip to the next open tab
- Ctrl+Shift+Tab to skip to the previous open tab
- Ctrl+1 to 8 to access the tab in those positions
- Ctrl+9 to go to the last open tab
I'm suspicious you are referring to going back to the tab you were on, after clicking a link that opens in a new tab. If so, it usually spawns in the next tab along, so going to the previous tab should do so. Otherwise, Ctrl+F4 / Ctrl+W will close the current tab, which should do so also.
A full list of keyboard shortcuts can be found here. I'm not sure a keyboard shortcut for exactly what you want to do exists.
Edit: I now understand how you mean, in that if you open a new tab, it will open as the last tab. I don't believe there is any shortcut (or any way to create a shortcut) to return you to the tab you were just on.