Who is responsible for my FMLA rights if my employer uses a staffing company?

The short answer is that your primary employer is responsible for your FMLA rights and the secondary employer is simply not allowed to interfere with your rights.

The Courts use different criteria for ascertaining who is your primary or secondary employer. Normally, for purposes of the FMLA, the primary employer is the staffing agency. Astonishingly enough, however, for FLSA purposes, i.e. overtime, the primary employer is the company that the employee provides services for, and not the staffing agency. This is particularly surprising because the FMLA was modeled after the FLSA. However, there is the Code of Federal Regulation that trumps this general rule: 29 C.F.R. 805.106 states that only the primary employer is responsible for providing FMLA leave and that normally the primary employer is the staffing agency.

Hence, if you are owed overtime, go after the employer you provide services for. If your FMLA rights have been breached, go after the staffing agency.