I installed the official "Firefox Multi-Account Container Tab" extension which "... is an extension developed by Mozilla that fully optimizes Firefox containers and unlocks its full potential[1]..."
It immediately bombarded me with multiple ads for Mozilla VPN, so I uninstalled it just as immediately.
The model here seems to be nerfing the built-in containers and locking away full functionality behind a confusingly similarly named extension to upsell you on paid services.
I put in an Amazon order, but the (virtual) card it's linked to no longer works because my online bank is horrible. Amazon requests that I revise the payment method, but won't let me choose the ACH method I set up, only enter another credit card number. I do not have another credit card number to enter.
So I have to create a new order with the same items, which does let me pick the new payment method, then cancel the old order.
You cannot cancel a whole order, you have to select the items one by one. There is no "select all" button.
The checkboxes to pick the items to cancel are hard up against the right edge of the page, and the scrollbar covers 80% of them as soon as you move the pointer there, making the click target about 4px wide.
If you try to cancel the order without selecting anything (which I did, because the checkboxes are very easy to miss), it resets the reason for cancelation. I didn't fall into this trap, but I bet it resets the checkboxes if you submit without changing the reason.