It fails to log in, it just comes back to the login page. I have tried clearing browsing data and cookies and that did not help. It works OK on Chrome.
Before trying @289wk’s suggestion, you might try simply turning Shields off for the site entirely and isolate whether or not Shields is having any effect on the behavior at all.
I regularly use USPS Informed Delivery, and have no trouble signing in with default shields settings (block trackers & ads, block fingerprinting and block cross-site cookies.)