Joining the 21st Century, if that's what this is

I finally got around to upgrading the theme on my website here. I really like the old theme, but that was 20 years old or so and no longer responsive to the new demands of cyber world.

Not that I need to, it just wouldn't come up on Firefox any more, or more accurately on my Mac 10.11.6 operating system.

This computer works perfectly, it's fast and has plenty of memory. Why am I forced to upgrade it to keep up with the glitterati?