Is this on your Mac? (Currently this post was slotted under iPad/iPhone; a moderator can move it for you.)

The payment processing site does not share data back to your browser, so you need to close that ad manually. You should find a close button to the upper right of the ad. I have attached a screen shot illustrating. It took me 3 tries, but I don't know whether that is typical.

I clicked on the x icon on the upper right-hand corner of the ad - to no avail. It does not go away.

I would appreciate it if you could figure out how to get rid of this.

Are you running Firefox in private browsing mode?

You posted with a user agent for a desktop Firefox version on Mac.

Can you confirm that this is a bout Firefox on a desktop computer?

Magic! The issue has been resolved. Thanks!