I don't get any pop-ups, just using a basic laptop and Firefox here.
Certainly is odd that you only see it on this site though, I have no idea why that is. You could think about running the freeware MalwareBytes cleaner just to be sure there's nothing lurking in your browser or system.
I see pop ups all the time but not that one. It is targeting you because you must have and use a smart phone (I don't have one). I get to see ads for roses and other gardening junk because that is what my wife looks at all the time.
This is a very common problem on phones. It has been widely reported on multiple forums I'm on, and three that I am a moderator on. It's not a site thing, it's an ad thing, specific to phones.
Hopefully this is not off topic but if the OP is using a phone I might have had a similar experience in the recent past.
About three months ago my 2-3 yr old Android phone started having a delayed pop up but I never noticed it particular to a website or any specific app. (don't recall what the advert. was)
First I deleted all of the picture 'photo-shop' apps I had recently downloaded. There was about 4-5 of them.
That didn't seem to do it so I deleted the apps from the months before that. All the common social media ones like Imgur, Tumblr, Instagram, Etc.
Then I loaded the phone version of Malwarebytes and I haven't had the pop up since then in over 30 days.
I've now reinstalled the social media apps about 2 weeks ago and still have had no pop up.