The `Allow pages to choose their own fonts` option was enabled. However, a customization in my userContent.css prevented the font changes to be applied:
pre {
font-family: "DejaVu Sans Mono" !important;
}
Starting Firefox with a new profile showed that the site works correctly.
For some reason, switching fonts does not work on my browser.
Firefox 64.0.2, windows 10 64 bits
No error message in the console, and ublock origin plugin didn't block anything on the page.
Opening the same page in Google Chrome worked well: font changes in the demo code whenever user clicks on a font in the list.