There is a hidden game in Firefox called Unicorn Pong and to play with it you need to know how to approach it.
Of course you can also play with Firefox Unicorn. Let's see how….
Step 1. First, open Firefox. Click the hamburger menu (the three horizontal bars) in the upper right, and then click "Customize."
Step 2. In the Customize Firefox tab, you'll see a list of available tools for configuring the toolbar. Click and drag all the toolbar items to the right of the "Overflow Menu" except the "Flexible Space".
Step 3. The "Unicorn" button will appear at the bottom of the window. Click on it.
Step 4. A Pong game with a small unicorn icon will appear on the left side of the tab. In this version of the game, the "Flexible Space" box serves as a Pong racket, and the unicorn icon is the ball.
To play, simply use the left and right arrow keys on your keyboard to move the UP flexible space so that the unicorn does not pass through it. The lower Pong packs are automatically handled by the game.
If you lose and want to play again, just double-click the Unicorn button.
When the game is finished, click "Reset Defaults" to reset all the items you went to in the Popup menu with Step 2.