That Click Me game is part of the Humble Mozilla Bundle promotion on the default Home Page [about:home], that loads from the Mozilla servers in the "Brand Logo" (DIV) section of about:home. A feature that was first used in Firefox 29 [IIRC] just prior to the start of the Olympic Games.

Also, part of that promotion is addressed in the "Snippet" below the Search Container on the about:home page.

If you prefer not to see the snippets on the about:home page or otherwise experience problems with a snippet then you can set the browser.aboutHomeSnippets.updateUrl pref to an empty string on the about:config page. Delete the storage\persistent\moz-safe-about+home folder in the Firefox profile folder with Firefox closed ("3-bar" Firefox menu button > Exit/Quit) to remove snippets stored in IndexedDB and make Firefox use a default snippet set. You can reset the browser.aboutHomeSnippets.updateUrl pref via the right-click context menu to the default value to re-enable the snippets and make Firefox recreate the moz-safe-about+home folder. You need to close and restart Firefox after changing the value of the browser.aboutHomeSnippets.updateUrl pref.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

• https://blog.mozilla.org/blog/2014/10/14/play-awesome-indie-games-directly-in-firefox-including-the-award-winning-ftl/
• https://www.humblebundle.com/?asmjs_bundle&utm_source=Firefox&utm_medium=Snippet1b&utm_campaign=Humble Mozilla Bundle