Why call a poem something completely unrelated to the content of the poem itself? Well, we're guessing that Millay intends us to think of the poem as part of a larger collection— and entitling it "First Fig" is a good way to make us curious about the second fig… and the third. That would keep us reading her poems, which is probably exactly what she wants, as well.
But ah, my foes, and oh, my friends– (2)
Notice how Millay includes interjections (Ah! Oh!) before she calls out to both the foes and the friends in this poem? That's a perfect way to make an address seem personal and informal while not actually making it directly personal. Interjections seem spontaneous—the sorts of words that you come up with when you're excited and want to point something out, or you're emotional and want to exclaim something out loud. And with two interjections, Millay manages to make both the friends and the foes seem equally important and included in the poem's message.
But ah, my foes, and oh, my friends– It gives a lovely light! (3-4)
After describing both the wonder of her brightly burning candle and its pitfalls (the dark night after it goes out, for starters), our speaker tells her audience what to think in no uncertain terms. She even includes an exclamation point so that we make no mistake about what we're supposed to think. Bright candles, however brief, are supposed to be awe-inspiring. And her confidence makes us pretty confident, as well.