A neat built-in feature that Chrome doesn't have but Firefox and Opera have is the concept of tab grouping. Sure, one can group tabs in windows, but pshaw, who wants that?
Regardless of my attempt at joking, I like Opera's better than Firefox, based on the virtue that I can see my groups, or rather stacks for Opera. Where Firefox groups it together in a "different" UI and then disappear until summoned, Opera's behave like normal tabs, only stacked.
Yeah, Opera has stacked tabs for groups doesn't it? Firefox has that separate thing that makes it seem like multiple windows within the one window.