I've stumbled upon the word 火垂る, meaning firefly, in the Japanese title for Grave of the Fireflies. The thing is, there's already a kanji (蛍), pronounced the same way. The other weirded thing is that I can't find 火垂る in most dictionaries. On Jisho.org there's an entry but it's for the whole title, 火垂るの墓. There's even another, obsolete, kanji for ホタル, 螢, so I don't think it's merely an old writing.
What's the deal with these kanji? Why do every occurrence seem to be linked with the movie? Is it a novel, play-on-word writing or an older one? And why is it so hard to find in dictionaries?