To better understand all of the symbolism and themes, one has to put this into context. Will Graham, now free from incarceration via Hannibal attempting to frame him, leading to a situation where Hannibal nearly killed Will and has fled to Europe, has woken up and is now traveling to find Hannibal, which he recently had in Italy prior to this.

Their relationship is an ongoing cat and mouse game, and given Will's experiences with Hannibal, being able to understand his mind, and the delirious state he woke up in after the season 2 finale, the audience is no longer 100% sure of what Will's motives are in hunting Hannibal. Does he want to bring him to justice, compete with him and become a greater serial killer, or run away with him? That is part of the intrigue of season three.

1. Metamorphosis: The Firefly Man Tableau is immediately about Will "evolving" being able to "reach" Hannibal's level of creation ("this is my design") by being able to create something so artful. He has taken the time to travel to Hannibal's childhood home and snoop around into Hannibal's upbringing. So by doing this here, using a person who allegedly killed Hannibal's sister, and being inspired by the fireflies on Hannibal's grounds, he has basically created a calling card, invitation, or love letter to Hannibal. Will Graham is invested in going deeper in uncovering the history of Hannibal.
2. Foreshadowing & Homage: The Firefly Man is also symbolic for anyone who knows the source material as the structure or appearance is similar to the Death's-head hawkmoth featured in Silence of the Lambs (book & film adaptation).
3. Spiritualism: Now while it's true, we do not know exactly how The Firefly Man will be foreshadowed, because the series was not able to finish and it had yet to introduce Clarice Starling, we may be able to read into the symbolism associated with the Japanese spiritual cultural context of Fireflies, also called "Hotaru" since the writers also chose to introduce an adult version of Chiyoh form Hannibal Rising here as well. Hotaru is a metaphor for "Passionate Love" poetically speaking, but is also is sometimes thought to represent the souls of soldiers who have fallen in war. In fact, there is an iconic Japanese animated film titled, Grave of the Fireflies that specifically also deals with these concepts. Given like Hannibal Rising, Grave of the Fireflies that is also a post WWII story about two orphaned children who were not able to survive long after the bombings of Japan, it's also easy to see potential parallels to the romance/companionship between Hannibal and Will, Hannibal, Will and Abigail, etc all echoing the loss of Hannibal's sister Misha. However the timeline of the TV series is updated and thus this version of Hannibal's story can not have same WWII backdrop of the source material. It can only be seen as homage to the source material, rather than the full vision of TV series itself.