May I fire someone for complaining about working conditions?

No. Employees have the right to discuss working conditions with other employees and are protected against discipline or termination by Labor Code Section 232.5 and Section 7 of the National Labor Relations Act, which applies to both union and non-union employers. California's Fair Pay Act also protects the right of employees to discuss wages and inquire about the wages of others.

Employers may not require, as a condition of employment, that employees refrain from discussing working conditions or wages. Neither may an employer require that an employee sign a waiver of that right, nor discharge, formally discipline or otherwise discriminate