I don't think it would work for me. My Firefox windows are 1366 pixels wide and screen height tall on a 4K screen. I don't have any sidebar space to waste. Maybe it's OK for people who maximize their browser windows on 16:9 screens.
I generally size my browser so that the viewport is roughly square - a wide squat browser usually leaves acres of margin, or on sites that don't have decent margin, enormously long lines that are all but impossible to read.
A square viewport on a widescreen monitor leaves loads of space for a sidebar.
This is how I do it, too. Given those 16:9 monitors, moving the tabs to the side is definitely an improvement in readability and efficient space usage.
