Not sure what's going on here, never ran into this problem before.
I locally built a test index.html page and linked it to an external css file and firefox quantum, firefox 56, edge, IE, opera are not displaying the css. I even dropped into my WAMP test bed and it still didn't work.
Browsers: Firefox Quantum
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<link rel="stylesheet" type="text/css" href="style.css">