Sortable rich text areas broken in Firefox

Using the example code below:

``` php
'sortable_wysiwyg',
'limit' => 0,
'sortable' => true,
) );

$field->add_meta_box( 'Sortable wysiwyg test', 'post' );

$group = new \Fieldmanager_Group( array(
'name' => 'sortable_wysiwyg_group',
'limit' => 0,
'sortable' => true,
'children' => array(
'text' => new \Fieldmanager_RichTextArea(),
),
) );

$group->add_meta_box( 'Sortable wysiwyg in a group test', 'post' );

} );
```

The following behaviour is observed in the latest Firefox:
1. Clicking 'Add field' for the single field results in a broken rich text field
2. Clicking 'Add group' for the rich text field in a group works, however when reordering the group the text from the rich text area in the group that was moved disappears