This is the 2687th most frequent Spanish word.
firmó
"Firmó" means "he/she/it signed" in English.
Here, 'firmó' is the past tense form of the verb 'firmar', mean to 'sign'. It is used to indicate an action performed in the past: the act of signing the treaty in this context.
El presidente firmó el tratado de paz.
The president signed the peace treaty.
In this question, 'firmó' is again the past tense form of 'firmar'. It is used to inquire whether the action of signing the letter happened in the past.
¿Ella firmó la carta finalmente?
Did she finally sign the letter?
Once more, 'firmó' illustrates the completed action in the past, specifically the signing of a significant document, in this case, a contract.