Firefox 52-x error "Access Denied You don't have permission to access...this site...on this server. Reference#...
This error shows only on Firefox 52 to the newest version, and happens whenever I click to check out on some shopping websites, like Costa.co.uk and currys.co.uk.
For troubleshooting purposes, I copied my portable Firefox setup and added a gift card and filled in my email address on Costa website:
I checked to check out, but the error page shows instead of allowing me to proceed to payment.