They removed that feature. Why? Good question. That said, you can get the same functionality by right-clicking on the back button (which you could also do in 3.X) Alternately, you can install the following add-on ... http://kuix.de/mozilla/backforedrop/ ... which will add a drop-down button next to the back/forward button. Sadly, the drop down button is much bigger and thus more intrusive than the old one, but it's all that's out there.

You can also hold down the left mouse button on an enabled Back or Forward button until the list opens.