The duties of an employee to his employers are essentially the same. And breaching those obligation is valid cause to be sacked. The user you replied to wrote:

The duties of an employee to his employers are essentially the same. And breaching those obligation is valid cause to be sacked. The user you replied to wrote: "Of course, you're free to speak ill of your employer at any time. And they're free to fire you for anything they hear about." That's broadly how it is in France (there is obviously a threshold with regards to what you say) Quick Google: "Un salarié critiquant violemment la politique des dirigeants de son entreprise et dénigrant son employeur ainsi que d'autres salariés. Faute grave. (Chambre sociale, arrêt n° 10-16929 du 27 septembre 2011)"