For landscape browsing (all PCs, and some mobile users prefer it), to be able to switch to side tabs is invaluable.
It was integrated part of Chrome in its early versions, and was dropped for some reason, and I still use it on my firefox.
I do not think it should be a very “big deal” to add such an option.
I use such a setup even browsing my gmail on my mobile, since I maintain strict filtering policies with multiple nested boxes for different topics and subtopics. So Portrait-wise, I know it works a treat.
My “two cents”