The poem sets two images side by side without explaining the link. First comes an "office fire," a sudden, human emergency. Then, on the road, a long line of ants moving in convoy, each one purposeful and in step with the others. The title, "Formic," points to ants, so the poem quietly asks us to look at them closely.
The last two lines turn to the speaker. "I have more mind / to fit in a nest" is a slightly twisted version of the idea of having a lot on your mind. Here the mind is something too big for a small, crowded home. Judging by the imagery, the speaker may feel crammed in by office life, or envy the ants' simple, collective order, or both. The fire suggests work as something that burns people up.
The mood is dry, wry, and a little restless. It's funny on the surface, but there's an ache underneath: a sense of not quite fitting the space you've been given.