I was able to fix this on my Pixel 2 by going to Settings > Google Services & Preferences > Ads > Turned on opt out of ads personalization, and I also reset my advertising ID for good measure. Was happening in both Firefox and Chrome. I could not figure out how to stop this. I downloaded Malwarebytes and other anti-virus apps, reset app cache on both Firefox and Chrome, uninstalled apps that I thought might be the culprit, and even factory reset my phone, but it kept happening. Finally I came across this forum post over at Google's Help Forum:
https://productforums.google.com/forum/#!topic/chrome/cCJTpaM-AhI
Basically scam advertisers are hijacking legit ads. Quoting from the post: "This is caused by a fraudulent google adwords advertiser using remarketing to follow us around sites that display adsense ads. They are using some kind of manipulative ad (which auto redirects to these sites without even clicking it). They appear to be cloaking the final url so Google don't notice"
Mind you this post is two years old and this crap is still happening, but after doing what I described above, so far I have not had any issues. I suggest reading AS797's other comments to get further understanding.