Final answer:
The quote in question is spoken by Macbeth in Act 4, Scene 1 of Shakespeare's play, where he firmly decides to act on his immediate impulses.
Step-by-step explanation:
The quote "From this moment the very firstlings of my heart shall be the firstlings of my hand" is spoken by Macbeth in Act 4, Scene 1 of Shakespeare's play. After Macbeth encounters the witches for a second time and hears their prophecies, he makes a resolution that from then on, he will act on his immediate thoughts and impulses. This declaration signals a shift in Macbeth's character to a more resolute and action-driven approach, as he no longer deliberates or contemplates the consequences of his actions as he did earlier in the play.