The UI of Firefox is actually drawn using all web technologies. The tabs were completely rewritten when changing the styling. The old tabs used a border that w

The UI of Firefox is actually drawn using all web technologies. The tabs were completely rewritten when changing the styling. The old tabs used a border that was drawn along the edges, which turns out to be quite expensive. Another expensive implementation detail in the old tab shape was a subtle linear gradient. The border and gradient are computed each time that they are drawn, whereas the new tab design uses a combination of background-images and SVG clip-paths (the clip-paths are used to get mouse events to follow the curved shape of the tab).