Former EMS provider...
The laws on whether ambulances must stop at red lights and stop signs vary by locality.
It's almost always legally your mess if you don't stop at a traffic control device and are then involved in a collision — most laws and various insurance or service policy requirements tend to require "due care" or "prudent" or other such wording, and collisions are usually considered an adverse indication.
I've always wanted the traffic signal in front of me to be green on approach.
This reduces the numbers of stopped cars in my path.
Some of the drivers of the stopped cars at intersections will attempt or will perform odd or normally-unexpected maneuvers, sometimes leading to collisions. But mostly the herd in front of me scatters short distances, and generally resulting in intersection gridlock.
It's common practice in some areas to avoid using the siren in certain situations, such as when passing to the right of the bulk of (stopped) traffic waiting on a red, using the breakdown or turn lane at an intersection, for instance. Some folks will hear the siren and immediately block that lane, or they might turn into and collide with me.
The local traffic preemption systems all include a flashing indicator — a rotating red beacon on a cross-arm or a pole, for instance — indicating that the preemption request has been accepted and is occurring. This tells me that the other lights are all red. Or it tells me that I'm approaching a non-preempted light, and that the green in front of me might go red.
If the traffic lights are red and there's no preemption and no open and no bypass lane, then some of the folks at the front of the herd will inevitably not break that red, meaning local traffic gridlock, meaning that the ambulance waits (with the siren off) for the light to change.