Hi John

This does not sound like an issue with Firefox but what happens when you try to add a credit card to your ASDA account? Is there any error message?

For your information I have solved the problem, but I have no idea why the process did not work on my PC, I used my laptop running on all the same programs as my PC and I have been able to get my bank card attached to my account. Thank you for your attention in this problem.

No problem, glad to hear it has been resolved.