Does it still happen in Troubleshoot Mode?

Thanks for responding.

No, it doesn't happen. Troubleshoot Mode signs me out of accounts, and it only re-starts when I sign in. Subscription websites then go into ad-free mode, which presumably alters the way they interact with browsers? What were ads and pop-ups sometimes become moving boxes.

One of the most troublesome instances is on newspaper sites that have a live blog with reader comments enabled. The whole of the lower half of the screen (where the comments are) turns into a grey box with left-to-right movement, so I can never read down to the last few posts. I use these blogs a lot for news coverage because they're so much easier than TV news programmes with scrolling tickers on the screen.

This issue sometimes doesn't make much sense to non-autistic people, until I explain that reading in the presence of moving content is like trying to finalise your tax accounts whilst being whacked on the head by a toddler with a cardboard tube. It doesn't take long to reach the point where you *really* need it to stop!

Thanks again.

Further information:

I've realised there's something else going on as well. In Firefox, one of the news sites I use always shows "Sign in" links on various parts of the page. I am signed in, and when I click the links the site recognises this, but the invitation to sign in remains visible. This doesn't happen with other browsers.

The attached screenshot shows one of the layouts I see when signed in. All of the grey boxes, which would normally show comments etc, are rapidly scrolling. The screenshot has captured the movement as lighter areas of colour.

That's a loading animation that would normally go away once the content has loaded. Your extensions are most likely blocking content, try disabling them.