The Firefox homepage is now displaying a game with 'Click Me' call to action button. How to change it back to normal (previous) homepage?

This is by design. Mozilla is supporting a Humble Bundle promotion right now.
Someone asked this question on the Mozilla support forums and received the following response:
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 want to block the "Brand Logo" section (and the "snippet" below the Search container) from ever being seen again, you can install Stylish and this UserStyle. https://userstyles.org/styles/104673/about-home-hide-snippets-and-hide-brand-logo
Another solution was offered for this as well:
Type about:config in the address bar.
On the first column, find the browser.newtab.url value. Double click it and change it at will. The default value is about:newtab, if you don't know what to write in it.
That being said, you should check and remove any suspicious extensions and plugins (such as toolbars installed by third party software). You should find this as a puzzle piece in the menu button, to the right of the search engine bar.
Use www.google.ca/firefox or www.google.com.my/firefox and that will load the customary Firefox start page on Google. One is in Canada, the other is from Malaysia.
Change default start page to get rid of Humble
In Mozilla Firefox address bar write for example Google.com
Go to Options, choose General, click on Use Current Page
Then click OK
From now on you will have Google.com as START PAGE and get rid of looking at the stupid Humble start page !!!