I assume firefighters are not unionized in California? Otherwise I imagine the union getting pretty upset that unskilled, non union members are being paid to do the job of union members.
The IAFF has 30,254 members as of 2015 per the Office of Labor Management Standards and they are roughly 1.1 million firefighters in the US.
Ultimately, my department decided not to unionize. I don't actually recall firefighter I worked with from my area being a member of a union as part of being a firefighter, most of them were union through ambulance companies.
When fire season starts in California, you couldn't produce enough hands to get the job done at basically any price.
When I was a seasonal firefighter for Cal Fire (CDF at the time), every figherfighter had some level of union support, but there was "Full Share" and "Fair Share" options.