Do firefighters get std/hiv tested every month in oroville/butte county?

Unlikely. No insurer or employer could afford to do this with that frequency, and there would not be much benefit to it. Ems and healthcare workers are usually tested only after a known exposure / event, which can be years in between if they are using proper safety precautions.
That seems like. A very short interval. You should call the health department but most first responders get tested no more often then yearly and not sure why this cohort of people would need standard std testing at all.