In bug 439354 I changed the unified toolbar to use a -moz-appearance instead of a background image for the gradient. This made the inactive window style for the toolbar background unnecessary: -#main-window:not([active="true"]) > #navigator-toolbox > #nav-bar { - background-image: url("chrome://global/skin/toolbar/toolbar-background-inactive.png"); -} However, removing this rule resulted in a 10% twinopen regression. I put it back in changeset ff469575e3ee. In Stefan's words (on IRC): > As it is now, this basically means "yeah, use '-moz-mac-unifiedtoolbar', > but make sure to add css background style in order to avoid a 10% perf hit" In fact, it doesn't make sense at all that a background style changes anything for a themed frame. We should find out why this is the case and fix it properly.

that is very strange!

I think fixing bug 453541 will fix this.

This is probably not an issue any more.