"overflow: auto" should always display a vertical scrollbar, though that scrollbar should be disabled when there is no need to scroll vertically. "Popping in" a vertical scroll bar causes a reflow, which is disorienting to the reader.

overflow:auto is fine the way it is. If you want to control the appearance of the scrollbars, use the -mox extensions to the overflow property: -moz-scrollbars-none -moz-scrollbars-horizontal -moz-scrollbars-vertical

Well, I don't agree that the current interpretation of "overflow: auto" is the most usable one, but whatever.... The problem here (in terms of making the browser do something desirable) is that there doesn't appear to be a way to have a vertical scrollbar that is visually present but *disabled* until there is an actual need to scroll. Why would anyone want that? To prevent Unsightly Reflow, of course. AFAICT, the Mozilla-specific values you mention don't help with this. I've filed bug 76197 on this issue.