You may have to set preferences in about:config. That site doesn't autoplay here. media.autoplay.default = 5 [0:allow;1:blockAudible;5:blockAll] media.autoplay.block-event.enabled = true media.block-autoplay-until-in-foreground = false media.autoplay.allow-extension-background-pages = false media.autoplay.blocking_policy = 2

https://support.mozilla.org/en-US/questions/1378457 https://support.mozilla.org/en-US/kb/about-config-editor-firefox

You can look at these prefs on the about:config page to see what settings work for you to block autoplay.

• media.autoplay.default = 5 [0:allow;1:blockAudible;5:blockAll]
• media.autoplay.blocking_policy = 2
• media.autoplay.allow-extension-background-pages = false
• media.autoplay.block-event.enabled = true
• media.block-autoplay-until-in-foreground

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

• https://support.mozilla.org/en-US/kb/block-autoplay

Thanks Terry and cor-el! It works now. It appears that the UI setting is broken?