Restarting the browser can fix that specific error; it typically results from a failed TLS handshake, and the failure is "remembered" until the browser restarts.

But in the not-too-distant future, you'll be living in a world where Firefox can't log you in to either aa.com or usairways.com, because both are using outdated, insecure SSL/TLS setups and Firefox will disallow the connection (or you can be like me, using the developer edition of Firefox, and already live in a world where Firefox can't log you in on either site; that stuff trickles into the preview and dev editions before it goes to full public release).