Custom CSS on Firefox (linux) with Refined GitHub

I am using Refined GitHub and have this snipet as a custom CSS:

/* optional, you can also just use system fonts. */
.react-code-text, .react-code-lines, pre, code, .blob-code, .blob-code-marker {
  font-family: "Monaspace Xenon", monospace !important;
  font-size: 14px;
}
pre.CodeMirror-line {
  font: inherit;
}

On Chrome, this works fine. I get the Monaspace Xenon font.
On Firefox, no luck. I just get the default monospace font.

What am I doing wrong?

githubnext / monaspace

Custom CSS on Firefox (linux) with Refined GitHub #128