Forums → Games → Hunter x online in Firfox browser

I don't use Firfox browser, but over the past few days it's been periodically popping up a window. The site is a signup page for what looks like an online game names ""Hunter X Online"". This is the exact site, obviously, go there at your own risk: http://hunter.unigame.me/

I've uninstalled all recently installed applications, and searched online for a solution. It seems to be due to known adware that is years old, and all reputable pages recommend removal with Malwarebytes Anti Malware, which for me (with a subscription and latest definitions) has failed to detect it with multiple scans.

So right now I'm kind of stuck. I'm sure I can get it to stop popping up in Firfox by playing around a bit, but I'd prefer to remove the offending adware altogether rather than treat the symptoms.

Thanks in advance for any help or info!