Malicious ads?

I'm on Firefox on Windows 10. My machines are pretty clean, so I highly doubt it's malware on my machine.

I opened a thread in a new tab (safeway-gap-gift-card-20-off-2123340/), and Firefox went haywire, opening new screens with the same red webpage, with a klaxon/siren going off, a JavaScript popup requesting my username and password, and some message about how something tried to access my bank accounts, but my firewall stopped it. Obviously all fake, but I wish I took a screenshot so I could have posted it here.

I haven't been able to replicate it yet, but if I do, I will update this post.