As these promotions are showcasing what's currently are going on from mozilla to teach the web and whats new with us. So, once the campaign done with its goal , it automatically leaves from the main page. However you can customize your own web page from here :

• Menu>options>General : Startup > Choose My homepage and set any favorites there.

Check out this UserStyle. https://userstyles.org/styles/104673/about-home-hide-snippets-and-hide-brand-logo

If you prefer not to see snippets on the about:home page or if there are problems with a snippet then you can set this pref to an empty string value on the about:config page by deleting the current value via the right-click context menu (Modify) or a double-click.

• about:config page: browser.aboutHomeSnippets.updateUrl (delete current value)

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.

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)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

That is an advanced setting; image.animation_mode = once

Type about:config<Enter> in the address bar. If a warning screen comes up, press the Be Careful button. This is where Firefox finds information it needs to run. At the top of the screen is a search bar. Type image.animation_mode