[xorcist]

I believe the UI of Chrome is noticeably more snappy, which largely contributes to the feeling of it being speedier. The PR was about how their Javascript engine was faster, but I doubt that was what really made any difference for most people.

The Firefox suite was written in XUL, which is Electron-like but faster and especially tailored for desktop software. It's fast, but not as fast as native UI. Given the choice, almost everyone went with the speedier UI. At least that was my completely unscientific feeling back then. There's probably something to learn about UX here that very few people care about.