1. The fact that they look like buttons now, not tabs. 2. The active and inactive tab colors are so close that it's difficult to tell at a glance which is whic

1. The fact that they look like buttons now, not tabs. 2. The active and inactive tab colors are so close that it's difficult to tell at a glance which is which. Especially in low-contrast situations (e.g. outside) or for those with vision disabilities. 3. Massive amounts of pointless padding. Which means you can't fit as many tabs in the window, and makes the browser a challenge to use for those with small screens. It also wastes horizontal space which is precious now that all monitors are widescreen. 4. The most important reason: they tried to fix something that wasn't broken and made it worse instead. There was nothing wrong with the Firefox UI but plenty of things they could improve where it comes to performance, extensions, HTML/CSS/Javascript behavior, and the large stack of open bugs in Bugzilla that have gone years without any attention. Instead, they paid someone (or possibly a team) hundreds of hours to fluff up the UI.