having issues accessing yahoo mail

I have a new HP laptop w/Windows 8.1. I downloaded Firefox and added Yahoo & Yahoo.Mail w/no problem. I needed to return the laptop to the service dept. to correct some issues and the technician cleaned the laptop to its original default settings. when I subsequently downloaded Firefox again I am unable to connect to my Yahoo.Mail. I have no problem accessing Yahoo.com When I attempt go to sign in to my account and try to view my mail, I get a security error message saying that I am accessing an untrusted website. additionally, the error message does not provide any solutions or exceptions to access my email. Please provide a solution or exception so that I can access my email via mozilla. Thanks, Les