The fallout from Anna's death, especially when it comes to Mal and Cameron's relationship, which is pretty much ruined.
Eiryss/Fire meeting Cameron. She tried to remind herself that she's going by Eiryss and she's not related to him, but she failed. Worse, seeing the girl who was just a few moment ago capable of taking down her own Alternate by herself reduced to a little girl clinging to her father was just ... heart breaking.
Both versions of Fire in general are this, to be honest.
The end of chapter fifty-one. One has to disguise a sob as a giggle, and it's hard to tell whether the sob was because it was so awesome, or just because Anna.