This blog post covers just one of the many performance tests that we ran while working on the version of Firefox. We also tracked other key metrics such as new window creation and the amount of time to enter/exit customization mode.
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).
