It may not work because there is no longer a Status bar at the bottom of the page.
See if it works if you make the Add-ons bar visible: View > Toolbars > Add-on Bar

You are correct, when you turn the Add-On bar on, it works again. Almost seems like a bug to me as the bottom of the window is well, the bottom of the window regardless if the bar is there or not. Hopefully someone who can fix it will happen upon this thread.

It appears the missing Add-on bar may have other repercussions. After using Search the selected files were displayed in Tile mode. I could select View>Details but the next search displayed in Tiles. After adding the Add-on bar the Details selection stayed on the Search Results.

This seems to be a bug in Firefox 4 and I'm sure they will probably fix it, but in the meantime, when selecting text that goes beyond the bottom of the page, I just hold my 'down arrow' cursor key down and it keeps on scrolling just as it used to do in the previous versions of Firefox.

It would seem, on OS X at least, that this issue was solved in the official release as it now works like you would expect. Not being a programmer, not sure it is a complicated issue that seems like it would be simple or if it was simply overlooked. Either way, problem solved.