You mean nightfall swirly head effect? Kinda blue/white-ish effect spinning around peoples heads? You get it (or could when I last played) by completing the weekly nightfall. Increases xp gains. You need to do the NF every week to keep it there. IF you mean literal flames that's probably an event mask (no stats, not an actual helmet) people are wearing. I think there was a skull and a fox type head that looked like they were on fire.