diff --git a/content/treestyletab/treestyletab.css b/content/treestyletab/treestyletab.css
index d4038a56..3a9275a0 100644
--- a/content/treestyletab/treestyletab.css
+++ b/content/treestyletab/treestyletab.css
@@ -25,6 +25,7 @@ tabbrowser[treestyletab-tabbar-autohide="true"] .tabbrowser-strip+splitter,
 
 tabbrowser:not([treestyletab-mode="horizontal"]) .tabbrowser-arrowscrollbox > .scrollbutton-up,
 tabbrowser:not([treestyletab-mode="horizontal"]) .tabbrowser-arrowscrollbox > .scrollbutton-down-stack,
+tabbrowser:not([treestyletab-mode="horizontal"]) .tabbrowser-arrowscrollbox > .scrollbutton-down, /* Firefox 3 */
 tabbrowser[treestyletab-hide-alltabs-button="true"] .tabs-alltabs-stack,
 tabbrowser[treestyletab-hide-alltabs-button="true"] .tabbrowser-arrowscrollbox + stack, /* "all tabs" button in Firefox 3 */
 tabbrowser[treestyletab-mode="vertical"] .tabs-closebutton-box,