This is weird. So you see the shield icon at the left end of the bar is a bit purplish? Firefox is blocking some tracking content. When I inspect the properties of that blank top section, it is something from that blocked site, commonly known as a tracking "pixel" (1 pixel square transparent image). Firefox is displaying it as a broken image, and then scaling it to a huge size, pushing the login form down the page. Disabling Tracking Protection (clicking the shield icon and then clicking the slider switch to unblock content) doesn't work a whole lot better. The page reloads and at first I see the login form, then the tracking pixel loads and pushes it down again. If you have any kind of relationship with that site, let them know there is something wrong with their page in Firefox. (They probably should not put their tracking pixel in that location, as the first item in a flex container.)