I use those mouse gesture browser extensions. Hold the right-click button, then swipe right or left. It's probably even more ergonomic than clicking those extra buttons (I have only ever used generic mice). Not that I am against extra buttons for other purposes, though.
I often have to run through folder systems on Windows so feature parity is important. At the office I have a mouse who has these buttons "hidden" on the scrollwheel. I can just push the wheel left or right. That's pretty ergonomic and actually feels quicker than having thumb buttons on the side of the mouse :)
I'm so used to it than I break down crying when I miss it. Unless I'm not using a mouse; if I'm using keyboard only I use keyboard shortcuts to achieve the same effect.