If you have the game paused in singleplayer, they build up. Resulting in an extremely laggy storm of fireworks the second you unpause it.

I was more concerned about server lag than singleplayer when I made this, tbh. I'll have a look at this before next New Year's ;)

Is it only for new years?

Yes, the fireworks are only for New Year's. Christmas has free energy with snow and multi-language season greetings on the RF bars. There's not yet a Easter egg for Easter ;)

Status?