Is the salary earned from working for a foreign company—from home and online—lawful (halal), knowing that the employee previously concealed the fact of his prior employment with them and applied under a different family name to circumvent the company's policy against rehiring former employees, and knowing that the company will terminate his employment as soon as it becomes aware of this?

Your deception of the company and your telling them other than the truth is not permissible, and it is a forbidden lie; because of the Almighty's saying: "So avoid the uncleanness of idols and avoid false statement," and because of the Prophet's (peace and blessings be upon him) saying: "Whoever cheats is not one of us." You must inform them of the truth of the matter. As for the salary you receive for the work you perform for the company, there is no blame upon you for it, provided that the work itself is permissible and you performed it as required.