I thought he meant Hagrid should stop trying to mediate between the Centaurs and get him to be accepted and forgiven by them.
From here we can see that Hagrid and Firenze knew each other as they met at least once before the 5th book(in the Forbidden Forest in the 1st book).
Firenze saved Harry Potter in the forest from Lord Voldemort,
frightening him away and carrying Harry on his back to safety. Despite
his heroics, his herd saw this as a dishonourable act, as they
considered themselves too great to be ridden by humans.
About four years later, in March 1996, Albus Dumbledore, Headmaster of
Hogwarts, hired him to teach Divination in Sybill Trelawney's stead,
after she was sacked by Dolores Umbridge. But once again, the other
centaurs saw this as servitude, and his colony attacked and banished
him. They would have killed him were it not for the intervention of
Bane: "Firenze! What are you doing? You have a human on your back!
Have you no shame? Are you a common mule?" Firenze: "Do you realise
who this is? This is the Potter boy. The quicker he leaves this
forest, the better." Bane: "What have you been telling him? Remember,
Firenze, we are sworn not to set ourselves against the heavens. Have
we not read what is to come in the movements of the planets? […]
Centaurs are concerned with what has been foretold! It is not our
business to run around like donkeys after stray humans in our forest!"
Firenze: "Do you not see that unicorn? Do you not understand why it
was killed? Or have the planets not let you in on that secret? I set
myself against what is lurking in this forest, Bane, yes, with humans
alongside me if I must." — Bane scolds Firenze for letting Harry ride
He was friendly with Hagrid, and seemed to get on quite well with
Harry, who was on speaking terms with him unlike most students, who
found him intimidating.
when the Centaur's herd turned on him for entering into "the servitude
of a human" by accepting the position at Hogwarts as a favour to
Headmaster Dumbledore, it was Hagrid who saved him.
Not canon, but my logic
I don't think unless the book absolutely says so, that Firenze would want Grawp abandoned like he was abandoned by his kind. Remember he has compassion, he came to the aid of Harry and he stood up for what he believed in (even though he had attempts on his life). Firenze it doesn't appear would have told Hagrid to kill his brother or back done, as he himself wasn't backing down.