Mozilla shows rotating snippets on the about:home page to run different campaigns and these changes regularly.

From within Firefox you can use these steps to disable and remove snippets on the about:home page:

• Set the browser.aboutHomeSnippets.updateUrl pref to an empty string on the about:config page
• Open the about:home page in a tab
• Open Web Console in this tab (Firefox menu button or Tools > Web Developer)
  https://developer.mozilla.org/Tools/Web_Console
• Run this command via the command prompt of the Web Console:
  gSnippetsMap.clear()
• Close the tab with the about:home page
• Close and restart Firefox

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

See also:

• https://developer.mozilla.org/Tools/Web_Console

It should change in a week or two. Try using a different home page. For example, if you generally search from the home page, try using the search site as your home page for the time being. These articles describe how you can set that:

• Startup, home page, tabs, and download settings
• How to set the home page

There 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