Fira Code appears to be one of them, at least that is used on the website. A handcrafted recommendation of mine would be to try Iosevka (https://typeof.net/Iosevka/ ) or Input (https://input.fontbureau.com/ ). I really like the narrow-width fonts, lets you fit more code on screen.

They're the ones I'm already using. I want to know what I'm missing out on! I've never seen Input though, so thanks for that.