Firefox

This browser needs a modest amount of feature creep just to reverse the past decade of feature retreat. I was mostly restricting the suggestions there to features that seem straightforward enough that they'd not lead to any cost to people who don't use them. It's by no means meant to be a comprehensive list. If you want one that would instead be very popular, how about this: https://connect.mozilla.org/t5/ideas/customizable-hotkeys/idi-p/4979