The statute says employees, contractors, and agents. This generally means that the protection only extends to the employer/employee relationship, whether W-2 or 1099. Outsiders, for example, have no protection under the statute. Nor do whistleblowers receive protection from those who were never their employers.
For example, if a potential employer refuses to hire you because you are a whistleblower, this provision does not apply. However, if you are fired by a current employer because they found out you blew the whistle on a previous employer, that would be a violation of the FCA.
Additionally, if you are the “associated other” of a whistleblower, such as a spouse or family member, your employer cannot retaliate against you as a way to get back at the whistleblower.