If you click the X then you place those items in the blocked list. That is the browser.newtabpage.blocked pref that you can inspect on the about:config page.

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

Note that Firefox still creates thumbnails of visited pages in the thumbnails folder when you disable the about:newtab page or aren't using it. You can create this Boolean pref on the about:config page to prevent Firefox from generating thumbnails for the about:newtab page.

• name: browser.pagethumbnails.capturing_disabled with value: true

You can delete the thumbnails folder in the Firefox profile folder to remove already stored web page thumbnails.

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

• https://developer.mozilla.org/Mozilla/Preferences/Preference_reference/browser.pagethumbnails.capturing_disabled
• http://mxr.mozilla.org/mozilla-release/source/browser/base/content/browser-thumbnails.js

Thanks for your help, but all I want is to pin my favorites websites on those 9 thumbnails that are occupied by some of my old pages that I don't want it.

You can drag a bookmark or history item on each of the 9 tiles. If you visit the link and have it in the cache then you should get a thumbnail in it.

You can change the order of the boxes on the about:newtab page and drag an item to a different tile to pin it to that position or you can remove a website by clicking the close X to block that URL. You can drag a bookmark or history item and drop them in a tile on the about:newtab page to pin them and have your own favorites sites showing.

Changes made manually on the about:newtab page are stored in the browser.newtabpage.pinned (default = [null]) and browser.newtabpage.blocked prefs that you can check/modify on the about:config page.

• https://support.mozilla.org/kb/new-tab-page-show-hide-and-customize-top-sites

Oh my god, thank you really much!