Theme toggle fails to inject in FireFox

[JJdeGroot]

Description of problem

URL to affected spec or repo:

See attached index.html file

What happened (e.g., it crashed)?:

The theme toggle does not display in FireFox.

Expected behavior (e.g., it shouldn't crash):

I expect to see the theme toggle.

Optional, steps to reproduce:

1. Open index.html in FireFox

Multiple errors are logged:

• Cross origin request blocked to load respect-highlight.js
• Error in updateTheme part of fixup.js line 374

Note that no error are logged when the webpage is accessed via: https://w3c.github.io/matf/

I would expect that respect-highlight.js should also be loadable from localhost.

And the updateTheme could potentially also work from localhost?
It does work in Chrome.