One thing that I would love to see in the Sailfish browser is something I miss from desktop Firefox - multi-account containers (or even a simpler way to deal with multiple profiles). I was quite dismayed, therefore, when after finally installing mobile Firefox via Android support I discovered it doesn't support it either (apparently it's desktop-only? no idea why).

On the topic of websites that do not work in the SFOS browser, that includes pretty much all Google websites; most notably for me, Google Voice and Google Maps. (I know the response in this forum will be "stop using Google products then" but that does not address the issue of compatibility.)