Firefox would not play embedded videos on Twitter. At first it displays the video’s preview image, but as soon as the video is loaded, it replaces the preview image with a black box containing the simple message
The media could not be played.
Now my browser has the FlashBlock add-on installed, and it could be the culprit.
So I checked the network traffic, and found the following domains to be involved:
- abs.twimg.com for static content, such as gif, css, js
- pbs.twimg.com for profile thumbnails
- video.twimg.com for mp4’s
Adding video.twimg.com to FlashBlock’s whitelist did not change the behavior.
Whichever whitelisting semantics is built into FlashBlock, adding twitter.com solved my problem, and embedded videos now also play on Twitter.