Basic moves are left, right, and jump. You can also grow or shrink your character with the keys alongside your jump key. For example, Q and E below.
If you're playing solo, you can use either side of the keyboard, or your arrow keys.
Playstation and Xbox controllers both work! This is 1000x better for head-to-head, so you can sit back and not cram over a keyboard.
Buuut...there are gamepad issues with Firefox. Please use Safari, Chrome, or MS Edge if you're connecting a gamepad.