According to jrmuizel, another thing that's hurting us during the customize mode transition for paint performance is border-radius's. There are extremely subtle border radii set on the toolbarbutton-icons, that I think we can temporarily disable during the transition.

Here's a full list of things with border-radius set on them: "back-button,toolbarbutton-icon,urlbar,notification-popup-box,searchbar-textbox,toolbarbutton-icon,dropmarker-icon,toolbarbutton-icon,toolbarbutton-icon,toolbarbutton-icon,toolbarbutton-icon"

(In reply to Mike Conley (:mconley) from comment #0) > There are extremely subtle border radii set on the toolbarbutton-icons That border is 100% transparent in the default button state (i.e. not hovered / open / checked). "Painting" this should be free.