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:
https://gifting.costa.co.uk/home/gift-card/thank-you-little-gc.aspx
I checked to check out, but the error page shows instead of allowing me to proceed to payment.
Clearing cookies/history, disabling, add-ons/running Firefox in troubleshoot mode, updating to newer Firefox doesn't solve the problem.
I tried deleting misc files and folders from Firefox directories and noticed that the problem is caused by: "content-prefs.sqlite" and "prefs.js", but after deleting those files or the profile folder, the error doesn't show on first load, but if I try a second time after clearing history, the error message starts showing again.
How do I fix this?