Go fish. Mobile should be safe from that crap, I would think. Two browsers also muddies the waters for troubleshooting it. Sorry. I'll think harder.
It isn't an uncommon problem. You can Google amazon pop-up rewards and get a lot of hits. It isn't really amazon it is some sort of malware coming from a site or apps. It popped up 3 times a few minutes ago while typing a long post in the PF. You have to shut the browser tab down. You can't get back to the website.
Seems like if I do all the above it makes it go away for a little while
Shutdown all browser tabs
Clear browser history
Shut down browser
Shut down wifi and turn on airplsne mode
Go to settings and clear browser history (again)
That works a little while but stay on this site long enough it comes back.