Must I confess my mistake concerning my colleague who was fired to the manager, knowing that my confession may lead to my dismissal and will not reinstate my colleague, especially given his bad behavior?

The questioner is not held accountable for what happened to his colleague because the dismissal was due to absence, and because the questioner did not complain about him. There is no justification for confessing a mistake if it does not yield any benefit, and it is not an expiation for the questioner's sin to be dismissed from his work as well.