Cannot open American Airlines web site

I cannot open the American Airlines web site (aa.com) using Firefox on my Windows 10 laptop. When I try, I get "Service Unavailable - DNS failure The server is temporarily unable to service your request." I have previously opened the aa.com site successfully using Firefox, but have now been unable for the last 3 days. I can open aa.com easily and successfully using Chrome and Edge on my laptop, and Firefox, Safari and the American Airlines app on my iPhone, so the problem is NOT with American Airlines.