source/Application/Model/Content.php
public function assign($dbRecord)
    {
        parent::assign($dbRecord);
        // workaround for firefox showing &lang= as &9001;= entity, mantis#0001272

        if ($this->oxcontents__oxcontent) {
            $this->oxcontents__oxcontent->setValue(str_replace('&lang=', '&amp;lang=', $this->oxcontents__oxcontent->value), \OxidEsales\Eshop\Core\Field::T_RAW);
        }
    }
public function assign($dbRecord)
    {
        parent::assign($dbRecord);
        // workaround for firefox showing &lang= as &9001;= entity, mantis#0001272

        if ($this->oxcontents__oxcontent) {
            $this->oxcontents__oxcontent->setValue(str_replace('&lang=', '&amp;lang=', $this->oxcontents__oxcontent->value), \OxidEsales\Eshop\Core\Field::T_RAW);
        }
    }
public function assign ( $ dbRecord ) { parent :: assign ( $ dbRecord ) ; // workaround for firefox showing &lang= as &9001;= entity, mantis#0001272 if ( $ this -> oxcontents__oxcontent ) { $ this -> oxcontents__oxcontent -> setValue ( str_replace ( '&lang=' , '&amp;lang=' , $ this -> oxcontents__oxcontent -> value ) , \ OxidEsales \ Eshop \ Core \ Field :: T_RAW ) ; } }
Replace the "&amp;" into "&" and call base class.

@param array $dbRecord database record
Replace the &amp ; into & and call base class .