Not the parent, but for Chrome, probably the horribly limited extension API. I could not even begin to imagine how I would use a browser without Pentadactyl. Or

Not the parent, but for Chrome, probably the horribly limited extension API. I could not even begin to imagine how I would use a browser without Pentadactyl. Or Tree Style Tabs. Both of which are flat out impossible in Chrome. As are tons of other extensions.