Denied access to sites via links or browser

Within the last 10 days my wife's computer has been denied access to a number of sites via the Firefox browser and she receives the Insecure Connection message. She uses the work around using the Advanced path button. We are confused why without any changes to her machine this is happening. She is on an iMAC using OS version 10.7.5 and Firefox 48.0.2 and accesses the internet through an internal network, she has Airport installed and is connected to my iMAC which has OS 10.9.5 and Firefox 78.13, and has never had any difficulties accessing sites or with Firefox. We're aware that our OS versions are outdated but all our applications work well and so we see no need to update. We'd appreciate any suggestions and thank you for your help. Frank B in Washington state.