If I could catch the green lantern of the firefly
I could see to write you a letter.
I remember hearing somewhere, I unfortunately cannot remember where, that the space between a poem's title and its body is like taking the step between the dock and the boat. That seems appropriate here. The title informs the "you" of the poem's text, presumably "a lover." A sense of longing permeates the poem. Sure, one can catch a firefly, but its light? Its green lantern? It will quickly go out, ephemeral as fireflies are. Lowell wants to write a letter, but can't see to it, the firefly's lantern being unobtainable.
Extreme short form poetry like this is attractive to me for its density of meaning and cleanness of form. Lowell says what she needs to say with no more than twenty words. And here I am, rambling on about it. I'm sure there are many interpretations possible from these twenty words; I only offer my own. I'd love to hear what you think about it, reader. Don't be shy.