What are illegal reasons for firing an employee in California?

In California, firing an employee can be illegal if it violates state or federal law. Common illegal reasons include:
Discrimination based on race, gender, age (40+), religion, disability, sexual orientation, or national origin (FEHA)
Retaliation for reporting harassment, unsafe conditions, wage violations, or discrimination (Labor Code §1102.5)
Whistleblowing or refusing to perform illegal acts
Exercising legal rights, like taking family or medical leave (CFRA/FMLA) or filing a workers’ compensation claim
Breach of employment contract or the implied covenant of good faith
Employees believing they were fired illegally may consult an employment attorney or file a complaint with the California Civil Rights Department (CRD).