This extension seems to break some websites as it hides all scrollbars within pages. For those who only want to hide the main scrollbar, this extention (https://cup.aihedy.com/en-US/firefox/addon/scrolantana) does exactly this and is hence compatible with all websites. It also makes inner scrollbars thinner, so they take less space than usual if this can help. Haven't found any issues with it so far.