connecting to needed app

As a retired American Airlines employee, I use fly.aa.com to book flights. I have been having a cell phone problem for the last year where it says "This connection is untrusted'. Go Back. Says owner of fly.aa.com configured website improperly. Firefox has not connected to this file. If I go there anyway, won't take my passworld. I use this app just fine on my pc at home, but need it to travel. A tech friend helped me go around once, but have not used it lately and it's back to the same screen as noted above. AA says its firefox problem.