Turns out it was something like this. The `Allow pages to choose their own fonts` option was enabled. However, a customization in my userContent.css prevented

Turns out it was something like this. 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.