whatslovegottodo, we can see that's actually an AdChoices advert - which are are pop-up ads based on your browsing history, using cookies. They are highlighted by a small blue triangle in the corner. These aren't a part of Mumsnet - so we aren't able to remove that, we're afraid.
It's an advert page for the new Xbox. It happens when I'm looking at threads on Mn, then this page pops up - I've put the page link in my post above. I'm on a chromebook: Version 61.0.3163.123 (Official Build) (64-bit) using Chrome. It hasn't happened today so far, but has been happening over the last few days.
The adverts are getting really very annoying now. I don't mind a small banner at the top, but EVERY time I click on a thread, half the screen (on a laptop!) is taken up by a massive ad at the top. Where I used to be able to read a full screen of comments on a thread, I can now only see a few comments and have to wait for the massive banner to disappear to see more. Even if I scroll down it takes a while for the advert to disappear.