You know, I was watching the airport footage again. There are a full 8 whole seconds between when the firetruck began to cross the runway (and the truck/s behind it all stopped btw...) before the plane hit it. Yes the truck behind it completely stops (like they should at a stop sign), probably because they saw the plane. As much as they got the clearance to cross, this sure seems like it's 90% the fault of the firetruck driver for crossing without looking first for crossing traffic...the size of an airplane.
https://www.reddit.com/r/airport/comments/1s1nhdm/air_canada_lga_crash_footage/