Firenze was likely predicting the larger war.
It’s unlikely that Firenze’s prediction was about the battle of Hogwarts specifically. As he explains to his Divination class, centaurs don’t tend to predict specific events in that way - they predict larger ‘tides’ of change or evil that may be marked in the stars.
“I, however, am here to explain the wisdom of centaurs, which is impersonal and impartial. We watch the skies for the great tides of evil or change that are sometimes marked there. It may take ten years to be sure of what we are seeing.”
- Harry Potter and the Order of the Phoenix, Chapter 27 (The Centaur and the Sneak)
Therefore, it’s much more likely that he was predicting the larger war against the Dark Lord than the occurrence any one specific battle. He also mentioned that the indications are that wizards were likely living through a period of calm between two wars, thus implying that his prediction is that there’ll be another war.
“Firenze pointed to the red star directly above Harry.
‘In the past decade, the indications have been that wizardkind is living through nothing more than a brief calm between two wars. Mars, bringer of battle, shines brightly above us, suggesting that the fight must soon break out again.”
- Harry Potter and the Order of the Phoenix, Chapter 27 (The Centaur and the Sneak)
The first war of the two he described in his prediction was presumably the war during the Dark Lord’s first rise and fall from power, ending when he attacked the Potters and lost his body.
“All right – Voldemort.’ Hagrid shuddered. ‘Don’ make me say it again. Anyway, this – this wizard, about twenty years ago now, started lookin’ fer followers. Got ’em, too – some were afraid, some just wanted a bit o’ his power, ’cause he was gettin’ himself power, all right. Dark days, Harry. Didn’t know who ter trust, didn’t dare get friendly with strange wizards or witches … Terrible things happened. He was takin’ over. ’Course, some stood up to him – an’ he killed ’em.”
- Harry Potter and the Philosopher's Stone, Chapter 4 (The Keeper of the Keys)
Therefore, it seems logical that the second war Firenze predicted was the Dark Lord’s second rise to power and the war that followed. After the Dark Lord appeared in the Ministry, Cornelius Fudge confirmed to the Muggle Prime Minister that they were at war.
“The point is, we’re at war, Prime Minister, and steps must be taken.’
‘At war?’ repeated the Prime Minister nervously. ‘Surely that’s a little bit of an overstatement?’
‘He Who Must Not Be Named has now been joined by those of his followers who broke out of Azkaban in January,’ said Fudge, speaking more and more rapidly, and twirling his bowler so fast that it was a lime-green blur. ‘Since they have moved into the open, they have been wreaking havoc.”
- Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Minister)
It seems highly likely that this second war was what Firenze had predicted, especially considering that centaurs predict large patterns rather than specific events, and that he’d already mentioned a calm between two wars, which are likely the two wars with the Dark Lord.