Multiple selection can be disabled in trees now.

xpfe/components/bookmarks/resources/bookmarks.xul

@@ -80,7 +80,7 @@
        style="width:100%;height:100%"
        ondragover="return DragOverTree(event);"
        ondraggesture="return BeginDragTree(event);"
-       ondragdrop="return DropOnTree(event);">
+       ondragdrop="return DropOnTree(event);" multiple="true">
 
   <template>
     <rule rdf:type="http://home.netscape.com/NC-rdf#BookmarkSeparator">