If this is a site you have visited frequently, you might have an accumulation of cookies that somehow conflict. Here's how you remove that site's cookies: First, save any pending work. Then, while viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm. Then try reloading the page (and, if needed, signing in again). Does that help?