You can see it in the livegrid example http://www.sencha.com/examples/#livegrid
If you increase the size of the scrollbar beyond 18 pixels, either by increasing the DPI/font settings or manually increasing the scrollbar size, it disappears in Firefox.
Firefox will refuse to draw a scrollbar if the div is too narrow to fit it, and the livescroller div is given a fixed width of 18px from gxt-all.css.
Removing/overriding this CSS rule will allow the scrollbars to show up, but there is still a lesser problem:
the column widths don't line up with the scrollbar gutter.
Investigating further, there seems to be 2 reasons for this:
First, inside GridView, the scrollOffset field is set to a hardcoded 19 pixels, when it should really be Math.max(19, XDOM.getScrollBarWidth()) as seen in a few other places.
Changing this alone does not completely fix the problem, because XDOM.getScrollBarWidth() is actually broken in Firefox and always returns 0 if the scrollbar size exceeds 25 pixels.