How to populate Quote data in form fields?

 $fieldset->addField(
        'first_name',
        'text',
        [
            'name' => 'FirstName',
            'label' => __('First Name'),
            'id' => 'FirstName',
            'title' => __('First Name'),
            'required' => true,
            'class' => 'input-text',
            'data-validate' => '{"required":true, "validate-FirstName":true}',
            'value' => $firstname // Set default value
        ]
    );