Ah Laurence, this is such an excellent haiku. I enjoy how It works on several levels.
I agree with Alan, the initial reading is painful, poignant. But reflecting on it, I also hold hope of a positive outcome.
I love how you have mirrored the on-off behaviour of the fireflies with the first two lines of the poem – juxtaposing a statement with a conditional, a positive and a negative – to convey poetically the experience of the on then off character of the marriage.
But we are left to ponder the outcome. Fireflies' lights are their method of attracting a mate, and the season is usually short. The poem might suggest the marriage resulted from a whirlwind summer courtship; the personalities of the couple struggling through the winter season. But then again, fireflies are also lights in the darkness, as another reader has commented. Fireflies are living embodiments of magic in the world, fantastic dreams that come true. The marriage may yet rekindle its magic.
As Basho says of great poetry, "The haiku that reveals seventy to eighty percent of its subject is good. Those that reveal fifty to sixty percent, we never tire of". Like a firefly dell, I think this is a poem to which we will never tire of returning.