Documentation Theme shows extra space between space tree branches in Firefox that doesn't appear in IE

Under 3.3, Documentation Theme has slightly larger gaps between items in the left-side tree than it did in 3.2, when displayed on Firefox. Seems trivial, but really makes a difference when the space tree has a lot of branches.

I've attached a PNG file which shows the difference.

Under 3.2, IE8 and Firefox show the exact same gaps between items.