All versions of the browser we currently know as Firefox have been supporting AES 256 bit ever since it was Firebird 0.6, May 2003 If the website you are using claims Firefox does not support even 128-bit then it may be incorrectly looking for the obsolete U that was in Firefox useragents before Firefox 4.0 to show sites it supported 128-bit at minimum. It should not need to since 2002 even. https://blog.mozilla.org/security/2014/10/14/the-poodle-attack-and-the-end-of-ssl-3-0/ Also since Firefox 37.0 the TLS 1.0 has been viewed as obsolete.