I had called T-Mobile about this problem while using FIrefox on their site maybe a year ago, and the problem is still happening with Firefox 57.0 (64-bit). It might not be possible for you to duplicate because it happens when I'm logged in and adding money to my T-Mobile prepaid account, but here's what happens at : https://manage.my.t-mobile.com/: 1) While logged in to my account, I click the Refill Now button to add money to my account. 2) That page lets me choose how I want to pay, like with a credit card, bank account, or refill card. 3) I click Credit Card, which takes me to a new page where I'd enter the necessary payment info, EXCEPT - that page only shows for a second or two, and then Firefox is back to the previous page. I've done nothing to make that happen, it just goes back to the previous page. This is exactly the same problem that previous versions of Firefox had, and I have no idea what it is on that page at T-Mobile. I worked around this issue simply by using another browser. With Edge, I can add funds to my account, no problem. I get no error messages in Firefox, it just doesn't stay on the page more than a second. If you have any ideas, I'd be interested in seeing if there's a fix. Or maybe it's my setup of Firefox, one of the options I've changed, But this problem happens at no other Web sites, so I'm guessing it's something in that T-Mobile page.