It has been a month that I was using firefox developer for playing mp4 video in ubuntu 20.04, because my vlc player not working at all. It's very good video player, but lack some features. For example, I need to slow down my player speed, but in firefox it has only option of speed 0.5x. Maybe they can add more slow down option, like 0.75x or it will be nice if there is an option that user can define it self what number of speed that suite to their needs. And maybe they can add an option to make the video more brigthness or more dark.