You have to give a example of the embedded YouTube video URL so others users can verify what is happening and if they get the same issues or not.

Thanks markwarner22

Below is a link to a website that I visit regularly. It often has embedded YouTube videos. I have never been required to be logged into YouTube in order to view these embedded videos in the past.

If you page-down there's a video entitled "TILT EQ USING FABFILTER PRO-EQ".

https://www.production-expert.com/production-expert-1/why-tilt-eq-is-your-friend

If you are using a VPN, it might refuse to play the video.

Having reviewed my Norton VPN Settings, I have discovered a 'Split Tunnel' feature that allows for apps to be added to an exclusion list, which removes the app from VPN traffic.

I have added firefox.exe to this list and whilst I can now play embedded videos when my VPN is running, this is not a real solution, as I no-longer benefiting from VPN anonymity and my entire online activity when using Firefox is traceable, which completely defeats the point of having a VPN in the first place.

It may be that YouTube have tightened-up their anti-bot security and this is the outcome. But it's noteworthy that they have not enforced this with their own Chrome browser.

If there is no way to view embedded YT videos within Firefox whist using a VPN, then it's a game-changed/show-stopper for me and I will have to move to a browser that doesn't have this issue.

So, any advise would be greatly appreciated : )

Problem Solved.

Traced the issue to my 'Privacy Badger' Extension. It was blocking 7 tracking cookies. I manually disabled all the cookies apart from the www.youtube.com cookie and the embedded video played.