tweetdeck.user.css

/* ==UserStyle==
@name        My TweetDeck
@namespace   https://gist.github.com/mono0x
@version     0.0.1
@license     MIT
==/UserStyle== */
@-moz-document domain("tweetdeck.twitter.com") {
  .column:first-child {
    width: 400px;
  }
}

youtube.user.css

/* ==UserStyle==
@name        My YouTube
@namespace   https://gist.github.com/mono0x
@version     0.0.1
@license     MIT
==/UserStyle== */
@-moz-document domain("www.youtube.com") {
  #menu ytd-button-renderer yt-formatted-string.ytd-button-renderer {
    display: none;
  }
  #menu ytd-download-button-renderer {
    display: none;
  }
}