some firms don't allow employees to give reference

A lot companies today have strict policies that any references have to be given through HR and HR typically doesn't comment on that person's performance but only employeement history. You can tell company C that your former boss in Company B told you that he won't be able to give you personal reference due to company policy and Company C can get your reference through HR of company B. In this way at least you won't get negative feedback from company B.