TL;DR: You don’t, he’s said his piece and you don’t need to comment or acknowledge it further.
Long version, you ain’t that special.
I will take it as a given that your performance last month was less than what you consider acceptable. That does not mean that you should have been fired. There are various non-humanitarian reasons not to fire an employee that does sub-par work. The most common reason is that over an extended time period their work is acceptable or even outstanding.
People are very seldom doing their best work while using the restroom, why don’t managers barge into the restroom and fire people? Two main obvious reasons, first and perhaps most importantly, they know that the downturn in productivity will be temporary. Secondly, it would negatively effect retention. If people are fired for a temporary and understandable downturn in productivity, other people will reasonably fear that the same thing will happen to them and they will start looking for employment elsewhere. Which leads to a less obvious reason, hiring people is expensive.
You may have seen movies or TV shows where guys in pickups drive up and hire “day-workers” with just a nod. That only works because what they are looking for is basically a back and a willingness to work. The most unskilled of “unskilled” labor. If it’s something more complex like taking orders and relaying them to a chef and then bringing the food to the customers, well, replacing that worker is going to take some time and money.
You don’t say what industry you are working in, and it almost doesn’t matter, firing you for a bad month would almost certainly, under the same theory of management, be grounds for firing your manager.
Unless the companies continuing operations rely upon you, then in the grand scheme of things, your off month just doesn’t matter. If it does matter, they need to reorganize things so that it doesn’t. You’ll see advice all over that indispensable employees should be fired ASAP. I wouldn’t say that, I would say that indispensable employees should be turned into replaceable employees as quickly as possible, and only fired if they fight that process.