The title is the key. Luciferase is the enzyme that makes fireflies and some fungi glow, and foxfire is the pale light given off by rotting wood. So the poem opens with a startling image: the self as that eerie glow, shining from something already dead. It suggests a sense of identity that is faint, borrowed, and living off decay.
From there the speaker moves, walking at night into ruins. The ruins could be literal, but they also feel like a picture of a worn-out inner world or a broken-down place where the speaker feels at home. The last lines are quietly odd. The walk is "toward a weary day's pleasure," as if even enjoyment has been used up before it arrives.
The mood is hushed, melancholy, and a little ghostly, but not purely despairing. A glow in the dark is still a glow. The poem seems to find a strange, dim beauty in being tired, small, and lit from within by something that shouldn't shine at all.