Once upon a time, Boeing Field Security and Boeing Field Fire were both part of a Department of Public Safety - type agency where all the Security folks were cross trained as ARFF folks.
We were never crossed trained as Firefighters at BFI. That was strictly the Boeing Fire Dept job. We did get dispatched to some of their calls but that was mostly to assist outside agencies (Seattle Fire, Tukwila Fire) to maneuver through the property when they were dispatched to the call. The city limits of Seattle and Tukwila runs through about the middle of Boeing Field dividing the north and south ends.
Boeing Security once upon a time did have special commissions thru the King County Sheriffs Dept. however they were commissioning all the Security people from Everett to Auburn/Fredrickson thru King County until they figured out the commission in King County is no good in Sno or Pierce counties so they dropped it in I believe the late 90's.
Boeing Field is technically The King County International Airport and as [im800mhz] said, the airport firefighters are King County Sheriffs deputies. They carry their bunker gear in the back of their patrol vehicles when doing LE duty.
The airport fire dept is the primary however they would let the Boeing people handle their own calls on company property unless otherwise requested. In instances where there were incidents like an A/C coming into the airport which was not a Boeing A/C and had declared an in flight emergency, King County would take care of it but keep Boeing Fire apprised of the situation in case they were needed. Boeing did have access to some of the Mutual Aid channels in the King County radio system but none of them, that I knew of, had access to the company radio system with the exception of Seattle Fire E27 there on Myrtle St. at Ellis Ave on the north end of the field and that was only because the company gave them a portable radio to use in the engine when responding to calls on company property....if they didn't lose it!
Security for all Boeing facilities in Puget Sound is now contract (rent a cop) security and has been since about 2017.
I know this because I worked Security for them for 27 yrs and the last 10 yrs (left in Jan 15) was at Boeing Field and the Duwamish Corridor.