Is the red light a flare?
yes it is
Who's firing it?
Warning the Ground Forces that an enemy plane/s approaching,balloons were the Radars of WW1.Red Flare for is for Enemy activity.
Oh I see, though some fighter pilots would use them to burn the balloon after a hail of bullets
The pilots not the guys up to the balloons Btw the pilots used specific Flares not a regular, something like the ballistic bullets ,i dont know how to explain to you but you can use the flare gun to shoot a ballistic flare.
Saw this on your youtube video. It's incredibly beautiful. I feel like I'm going to have a difficult time deciding which of your efforts will be my new desktop wallpaper.