Could you try the setting shown in the attached screenshot, which is, on the Options page:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the left column, click Privacy & Security

On the right side, scroll down to the Permissions section and check the box for "Block websites from automatically playing sound"

Then try YouTube again.

"Block websites from automatically playing sound" is checked (as already written in my original post.

Blocking autoplay on YouTube still works for me with media.autoplay.default = 1 (i.e. what I get with a checkmark in Options/Preferences).

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

resucami2 said

"Block websites from automatically playing sound" is checked (as already written in my original post.

Hmm, works for me. To be clear, it works on initial page load of a watch URL (such as https://www.youtube.com/watch?v=u0431tculpg). It doesn't stop videos from auto-playing during internal navigation when YouTube changes the page contents without a full new page load.

Thanks for your replies. I switched back to Firefox 65.0.2 and have no problems with autoplay anymore. I also disabled automatic updates and will again try a newer Firefox version in a couple of months.

After following the suggestions here some videos (with audio) on Youtube did not start automatically anymore, but some others (with audio) still did. I have no idea why and I don't think I will suffer when continuing to use Firefox 65.0.2 for some time.

It remains the question why Mozilla - and that not for the first time - disabled a working method to disable video autoplay (apparently also others have the same issue). From my viewpoint that is not an improvement - but what do I know, I'm just a simple user.

hmm...I'll have to check my settings but I don't recall YT auto playing unless I have the box in YT enabled but I loo those settings to see as well.

I have the Youtube "Autoplay" function disabled; this has no influence on some videos (with audio) starting automatically and others (with audio) not when using Firefox 66.

An example for a video (with audio) that did start automatically with Firefox 66 is this: https://www.youtube.com/watch?v=nsg9nQ03CdE.

Another example for a video (with audio) that did start automatically with Firefox 66 is this: https://www.youtube.com/watch?v=55Wety3Ut80

An example for a video (with audio) that did not start automatically with Firefox 66 is this: https://www.youtube.com/watch?v=cBKwvEizKZU.

None of the three YouTube videos you posted auto start for me. Although I see some flashing like ads are trying to play and are skipped automatically. I tested this in a clean profile without any Content Blocking extensions.

To make the "stop autoplay" work again use these settings:

media.autoplay.default: 1 media.autoplay.enabled.user-gestures-needed: false media.autoplay.allow-muted: false

Although the "media.autoplay.enabled.user-gestures-needed" seems to be wrong it brings back the "stop autoplay" feature.