All vehicles can override/ignore warning devices. Doesn't make it right. Emergency vehicles should not override/ignore train or plane crossings. Trains and Planes don't care about flashing lights. Crossing an active runway requires clearance for safety.

In this case, from the available information, the drivers of the fire truck thought they were cleared, and proceeded to cross while a plane was cleared to land. I'm not familiar with ATC ground radio to know if they were actually cleared or not, but it seems clear that that the drivers thought they were cleared.

Investigation reports will give us more details.