stylish-twitter.css

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document domain("twitter.com") {
  /* styles for collapsing media in tweets and showing an arrow to indicate their existence
     but only at the top level */
  ol.stream-items > .expanding-stream-item:not(.open) .OldMedia,
  ol.stream-items > .expanding-stream-item:not(.open) .AdaptiveMedia {
    display: none;
  }
  
  ol.stream-items > .expanding-stream-item .conversation-root > .has-content:before,
  ol.stream-items > .expanding-stream-item > .has-content:before,
  ol.stream-items > .js-pinned .user-pinned:before {
    display: block;
    position: absolute;
    bottom: .65em;
    right: 10px;
    color: #CCD6DD;
    font-size: 1.25rem;
  }
  
  ol.stream-items > .expanding-stream-item:hover > .has-content:before,
  ol.stream-items > .expanding-stream-item:focus > .has-content:before,
  ol.stream-items > .js-pinned:hover .user-pinned:before,
  ol.stream-items > .js-pinned:focus .user-pinned:before {
    color: #069;
  }
  
  ol.stream-items > .expanding-stream-item:not(.open) .conversation-root > .has-content:before,
  ol.stream-items > .expanding-stream-item:not(.open) > .has-content:before,
  ol.stream-items > .js-pinned .user-pinned:before {
    content: '\25BE';
  }
  
  ol.stream-items > .expanding-stream-item.open .conversation-root > .has-content:before,
  ol.stream-items > .expanding-stream-item.open > .has-content:before,
  ol.stream-items > .js-pinned.open .user-pinned:before {
    content: '\25BE';
    transform: rotate(180deg);
  }

  /* indents followup tweets in a conversation chain in the main timeline */
  ol.conversation-module li:not(.conversation-root) {padding-left: 1.5em;}
  
  /* So long, Moments tab */
  li.moments.js-moments-tab {display: none !important;}
}

Updated on userstyles.org https://userstyles.org/styles/120144/twitter-hide-auto-expanded-media-updated
(Wish I could remember my password so I could update/delete the original...)

Updated again, to account for threaded conversations that start with a media tweet (Twitter uses an :after to create the vertical bars to show reply threads, so I shifted it all to :before and added a selector).