I got this on Firefox (FF58). I fixed this with:
dom.moduleScripts.enabled
in about:config
Source: Import page on mozilla (See Browser compatibility)
type="module"
to your script tag where you import the js file<script type="module" src="appthatimports.js"></script>
./
, /
, ../
or http://
before)import * from "./mylib.js"
For more examples, this blog post is good.