The pong game being played in Firefox
Unicorn Pong is an easter egg in Firefox. To trigger the easter egg, you must open the Customize Firefox interface, and move every item except for Flexible Space into the Overflow Menu. Doing this will cause a unicorn emoji to appear near the other buttons. Clicking it will start a game of Pong, using two flexible spaces as paddles, and a unicorn as the "ball." You are given 5 lives, and the game ends once you lose all your lives. To play again after losing, double click the unicorn icon again. To remove the unicorn icon, move any items out of the Overflow Menu. Otherwise, just exit the menu by clicking Done in the bottom right corner of the Firefox menu.