The speaker loves someone, calls them "firefly," and decides never to say it. The reasoning is painful: the speaker sees their own "darkness" as a danger to a person who glows. Getting close would only "kill your glow," and the image of the firefly falling through the air, with teardrops scattered behind, makes that loss feel physical and sad.
The short dialogue at the end changes the tone. Until then, the speaker is alone with a private, almost resigned argument. Then the other person speaks, hurt and angry, and the speaker's calm answer, "You deserve better," sounds like a well-worn excuse for leaving. The reply, "I want you!", shows that the firefly doesn't see the speaker as a threat at all. The final "I know. I know." suggests the speaker feels the same but won't change course. The mood is tender, tragic, and a little self-punishing: love is being sacrificed to a belief that may be mistaken.