You need to use the [stylesheet_path] helper here so that you get the correct path when the application is deployed to production.

```erb
WebFont.load({
  custom: {
    families: ['MuseoSlab', 'icomoon'],
    urls: ["<%= stylesheet_path('fonts') %>"]
  }
});
```

[stylesheet_path]: http://api.rubyonrails.org/classes/ActionView/Helpers/AssetUrlHelper.html#method-i-stylesheet_path