Several reasons: - Until recently, Firefox did not support VA-API video acceleration on Linux, while mpv supported it for a long time. My last computer struggl

Several reasons: - Until recently, Firefox did not support VA-API video acceleration on Linux, while mpv supported it for a long time. My last computer struggled to play 1080p videos without hardware acceleration. - I am used to mpv keybindings, i do not want to use two completely different sets of keybindings for playing local videos and Youtube. - I like videos in a separate window (this feature Firefox started to support recently as PiP mode).