Oh! That white square on the right is supposed to be where the login form appears. I wonder why it broke for me in Firefox. 

 Must have been a temporary glitch, I just tried again (same browser, no change of settings) and got this:

https://cdn.masto.host/fedisimonwillisonnet/media_attachments/files/111/150/572/856/903/847/original/ae89aa0c7171ffc2.png