Any ideas why Twitter videos on this site have suddenly stopped playing on my machine?

[CloudWatcher]

8. Maybe, try this ....

I rarely use Firefox but spent a minute and reproduced the problem. Since I don't use Firefox often, it took the opportunity to brag to me about having blocked some sort of cross-site tracking with it's "Enhanced Tracking Protection." I'm all in favor of blocking cross-site tracking, but it seems to break watching Twitter videos (and animated GIFs) that are linked to by DU.

To fix ...

0) Go to a web page on DU that has a broken video
1) click on the shield symbol to the left of the address bar. You should see a popup window with "Enhanced Tracking Protection" at the top .. including a small toggle button to enable/disable it.
2) Toggle the switch at the top of the panel (turn it off).

This will allow cross-site tracking when Twitter is used from DU. Toggle it back on to revert to being protected.

Btw, the page will reload automatically when you hit the switch, but I had to do additional reloads before it worked.

Disclaimer: this worked for me, once, on a Mac running the current macOS. Your milage may vary.

Reference: https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop?as=u&utm_source=inproduct#w_what-to-do-if-a-site-seems-broken