STR: 1. Open https://mozilla.slack.com/messages/C4D3JFF26/convo/C4D3JFF26-1535390718.000100/ 2. Click to play the Bob Ross animation (from Slack's preview of https://twitter.com/firefox/status/1034121002748264454) RESULT: The animation preview is black.

The issue is reproducible and it is more likely related to `Images` breakage. Images link: https://pbs.twimg.com/tweet_video_thumb/De8RIV7UEAEVjQQ.jpg Video link: https://video.twimg.com/tweet_video/De8RIV7UEAEVjQQ.mp4 It is reproducible while Tracking Protection BASIC is enabled. [Prerequisites:] Slack account available and signed in. [Environment:] Browser / Version: Firefox Nightly 63.0a1 (2018-08-30) Operating System: Windows 10 Pro Looking at the devtools console, here are the blocked resources: The resource at “https://pbs.twimg.com/tweet_video_thumb/De8RIV7UEAEVjQQ.jpg” was blocked because content blocking is enabled. The resource at “https://video.twimg.com/tweet_video/De8RIV7UEAEVjQQ.mp4” was blocked because content blocking is enabled. So below are the domains to test: - pbs.twimg.com - video.twimg.com I opened the URL in a fresh browser profile (Firefox Nightly 63, uMatrix installed, normal mode) and loaded the page. An error is received and Slack is not loaded ("For some reason, Slack couldn't load"). I disabled the Spoof Referrer option in uMatrix and then WHITELISTED: - slack-edge.com (including all related domains) - slack-msgs.com and Slack started and the animation was displayed and when played a black preview was shown. I whitelisted: - twimg.com (including all related domains) and the animation previous was displayed and also plays when clicked. So in conclusion: - twimg.com is in Disconnect list = [tp-social]

Added screenshot with broken animation preview

Added uMatrix results.

Also reproducible on https://tweetdeck.twitter.com/.