What happened to https://js.arcgis.com ?!?!?
Same issue with
https://js.arcgis.com/3.19/init.js
https://js.arcgis.com/4.2/esri/css/main.css
They are all working fine for me.
Currently working for me, too.
I was getting errors like that yesterday for many of the esri sites in Firefox. I updated my Firefox and they seem to be ok today.