Yes,I've heard of hotaru.
Here's the info for it from site Frozen Call:
Chihiro will sing the theme of a Japanese film Last Game—Saigo no Sokeisen (The Last Baseball Game Between Waseda University and Keio University;
http://www.lastgame-movie.jp/)—which gives a picture of students who were killed in a battle during World War II—due out in August. This fantastic ballad, titled "Hotaru" (Firefly), has been picked up by the film's production staff from the collection of her unreleased songs. "Hotaru" is also expected to be released in August and will be sung live at the forthcoming concert this month.
also,if you go to
http://www.lastgame-movie.jp/ , you can hear the preview of the song on the video.
I really like the name of the song and I also can't wait for the movie <3