Both Classic and Modern don't have CSS for list* (listbox, listitem...) XUL tag that replace tree* (See view-source:jar:resource:/chrome/classic.jar!/skin/classic/global/tree.css and linkTree.css). This blocks the move from tree* to list* in XUL applications. For more information, on list* XUL tags, see bug #70858.
Not a blocker. Please read the severity descriptions....
This is a pointless bug, listbox isn't even implemented yet, and when it is implemented (bug 70858) the themes will arrive with it.