hello, please refer to https://www.ssllabs.com/ssltest/viewMyClient.html to see the cipher suites supported by firefox in descending order of preference. ECDHE-RSA-AES128-GCM-SHA256 identified to be in use on https://www.fortify.net/sslcheck.html is the second top on that list, so i'm unsure what you think the problem is....

THe issue is when I run the same test using forified https://www.fortify.net/sslcheck.html using Safari 9.3.1 it gives a result of "AES cipher, 256-bit key" the highest security value but firefox 48.02 does not have as high a security encryption.

bigger isn't always better in terms of security. if you want to have a discussion about this, i think the mozilla.dev.tech.crypto mailing list at https://www.mozilla.org/en-US/about/forums/#dev-tech-crypto would be a better place to discuss that (support.mozilla.org is a mere users helping users forum),

The highest encryption level is "AES cipher, 256-bit key" tested by Fortified. the lowest level is no encryption. Clear the highest encryption is the better since the number of bits "256" increases the encryption level.

Firefox 48.02 only shows "AES cipher, 128-bit key" compared to Safari 9.1.3 which tested "AES cipher, 256-bit key" which was performed on the same workstation and shows Safari with a higher leve of encrytion than Firefox.