How about showing the issue. Hard to help with something we cannot see. What version of Firefox? What OS? https://secure3.pennmedicine.org/EpicLink/common/epic_login.asp see screenshot

Penn Medicine advises against using Firefox for online doctor visits because it may not fully support video, audio, or security features required by their telehealth and billing systems. Platforms like MyChart, which handle both virtual visits and medical billing, work best on Chrome or Edge. Using Firefox could cause technical issues that disrupt your appointment or prevent billing information from being processed correctly. For a smoother visit and accurate billing, it’s best to use a supported browser.