Is it permissible to keep equipment belonging to a foreign company that went bankrupt in an Arab country, with no heir or responsible party, knowing that some are trying to seize it unlawfully?

The equipment in your possession from the company's assets is a trust (amanah) that must be returned to its rightful owners or their legal representatives. It is not permissible to give it to someone who will not deliver it to them. If the owners cannot be identified or reached, then this equipment should be spent for the general welfare of Muslims or sold, and its proceeds spent for that purpose. It is not permissible to seize it.