First of all, it's not permitted to use fireworks after the end of GRB because they make the lag that's bad enough at that time even worse; I don't know whether it's considered as a straight violation of ToS, it's probably not bannable or something, but for sure it doesn't help anyone.
Like Ridin, I am also pretty certain that there is no connection between guilds' GRB performance and fireworks. Seeing your name appearing on the log like e. g. '.Eadwacer. used fireworks' doesn't mean that this particular toon has really done that, it's a common glitch. Someone else just fired them and the program just messed the names. These log glitches seem to happen most with fireworks and in pvp combat. I often - sadly - see on pvp maps on my screen log messages like '.Eadwacer. killed .Eadwacer.'; still I am not aware of having suicidal tendencies of any kind:).