nicovideo-unsqueezed.user.css

/* ==UserStyle==
@name           Nicovideo Unsqueezed
@namespace      github.com/neofuji
@version        1.0.0
@license        CC-BY-SA-4.0
@description    Stretches the aspect ratio to 16:9 on NicoNico Video.
@author         Toru Higuruma (https://github.com/neofuji)
@homepageURL    https://github.com/neofuji/niconico-usercss
@supportURL     https://github.com/neofuji/niconico-usercss/issues
@updateURL      https://raw.githubusercontent.com/neofuji/niconico-usercss/master/nicovideo-unsqueezed.user.css
==/UserStyle== */

@-moz-document url-prefix("https://www.nicovideo.jp/watch/") {
    #MainVideoPlayer video {
        object-fit: fill;
    }
}