Could you try unchecking the "Suggested Sites" checkbox on the new tab page. See: Customize your New Tab page to hide or display content.

Does that do what you want to do?

If Firefox loses that setting at shutdown/startup, please see this article for suggestions: How to fix preferences that won't save.

If you want to remove the sponsored directory links on the about:newtab page then use these steps:

You can set the browser.newtabpage.directory.source pref to an empty string on the about:config page.

• browser.newtabpage.directory.source = <empty>
• browser.newtabpage.directory.ping = <empty>

You can delete the directoryLinks.json file in the parent location of the disk cache to remove the currently stored directory links.

• delete directoryLinks.json

See the about:cache page for the location of the disk cache and go one level up.

"AppData" (XP: Application Data; Mac: ~/Library; Linux: ~/.mozilla) is a hidden folder.

• http://kb.mozillazine.org/Show_hidden_files_and_folders