Reviewer Jesuotaku observed that the fireflies at the end at the same color as the lights of the city Kobe. These fireflies have little time of their lives, and Kobe would later be devastated by an earthquake...
Which would make this Harsher in Hindsight (relatively speaking), as the film came out in 1988 while the Great Hanshin Earthquake struck in 1995.
When Setsuko buries the fireflies, she asks Seita "Why must fireflies die so young?" while burying them. Most viewers thinking this would be the explanation for the title would be missing its true meaning. The children are the fireflies, and their impromptu home has allures similar to a tomb. "Grave of the Fireflies".
Invoked with its use of How We Got Here. The story starts with a train station guard finding the dead body of a young derelict and going through his meager possessions, which consist solely of a sweets tin containing some solidified grey-white powder that is promptly thrown away. It isn't until you watch the film that you realize what the powder is and why it was in the tin. For those who aren't familiar with the story (which you really should watch or read as long as you aren't scared of drowning in your own tears) — it's the ashes of the dead boy's younger sister, who he had cremated all by himself a few weeks previously. Worse because the police officer tastes it, probably believing it to be either food or some kind of narcotic.
Ya know, since the aunt was asking how come Seita and Setsuko don't find other relatives, how do we know those relatives are even alive? I mean, after all, Tokyo did get bombed about the time or a little after those two had to live with their aunt, so unknowingly and basically, she was sending them there to die.