I'm not sure it's well done in Firefox on Android either. It's based on assumption that you read each page once, and never scroll up. So scrolling up assumes you're finished and bringing up the toolbar is supposed to help you to navigate elsewhere. However for me - this doesn't work. I can go back and forth within the same page, and toolbar which pops up all the time can become really annoying. IMHO it's a trend of extreme minimalism which goes beyond what should be comfortably minimal.