Without clicking on it hover your mouse over the "Click for details" button, the URL you will be directed to should show at bottom left of window.
Thankyou for trying to help.
Perhaps some good by mysterious news is that the pop up window re Mcaffee virus not protecting my computer has not appeared when using the internet and the visiting the sites on which it did pop up earlier today and previous days.
I am not sure why that has happened but I did run a scan using Malwarebytes and isolated results that were of dubiouis origin but none that I could see were directly of the pop up window.
However, I also took up another contributor's suggestion of using an add pop up blocker. I could not see one on Firefox but did install their add on Privacy Badger so that may also have helped to resolve the issues.
So I trust the the problem is solved and therefore thank you all for your various ideas. Also to mention one other contributor who asked about whether MD Defender identified anything. It did not do so butas that contributor pointed out and I learnt something new I thought that I had turned Defencer off but it appeared to have modifiied itself so as not to conflict with my BT virus programmes etc but to my surprise was still working in the background.
I shall close the question with my thanks to you all
Happy to hear it's been resolved for you - hopefully all still ok. I had the same thing and it was still happening with the pop up blocker installed. I've tried AV scanning etc but still kept getting the annoying pop up. Noticed that ransomware and malware were misspelled in the alleged McAfee pop up so figured it wasn't genuine (and I don't have McAfee anyway). Anyway, long story short, I went into my installed Apps and found something called "fa really simple syndication (rss)". Given the pop up said Fast App store at the bottom I figured it might be related. Uninstalled it and had no problems since. Just in case anyone else has the same problem.