Hi! Have you tried reinstalling firefox?

No, reinstalling is not the first option due to all settings and plugins. But I'll do it if nothing else works...

Try this extension.

• Disable HTML5 Autoplay https://addons.mozilla.org/firefox/addon/disable-autoplay/

Thank you cor-el for the suggestion.

But sorry, this didn’t help. The videos are still auto-starting.

First link the videos don't play and second one the small videos seems to play and then stop as though doing a preview. So how do you mean Auto-starting?

Do you have other extensions installed that might be causing blocking autoplay not working?

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I decided to post my message in case this was a common problem or a bug. By now there should have been more users having the same problem if that were the case. (And there are in fact some postings which describe problems resembling my issue)

The issue started with the update to FF 66.0. But this may nevertheless be a problem in my local system. As soon as I get the time, I will save all my settings in my plugins and do what @aalppr asked and make a fresh install. If the problem remains, I will test if there are any conflicts in my loaded plugins (as indicated by @cor-el).

Thanks for all help so far!

I will let the problem be unresolved until I know more.

I made a fresh install of FF 66.0 without any active plugins. The problem remains and Firefox starts playing videos whenever a site enables it to play. Maybe this is a problem in my environment and that not all users are affected.

I have decided to downgrade to FF 55.0.2 until I am positive that the ability to stop videos from autoplaying is restored.

You can modify these prefs on the about:config page to see if that works for you..

• media.autoplay.default = 1 [0=Allowed, 1=Blocked, 2=Prompt]
• media.autoplay.enabled.user-gestures-needed = true
• media.autoplay.allow-muted = false

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.

• http://kb.mozillazine.org/about:config

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.

Thank you koooper for the solution! Now I am running FF 66.0.1 and I got rid of the annoying flickering videos.

(Thanks also to all the other helpful users posting tips and suggestions.)