"Top Sites" are determined by running a computation across all your currently stored history, combining the number of visits (frequency) and time (recency), to generate a score Firefox calls "frecency." Since this is constantly being recalculated, there's not really any place to clear the list, other than by more aggressively clearing your history.

But there are ways to hide this information. For example, you can pin bookmarked sites onto the page so that history-based sites do not appear. This article describes how you could do that: Customize your Firefox New Tab page.

Or you could display a completely different page on a new tab, using an add-on. For example:

• If you already use Classic Theme Restorer: There is an option somewhere (!) in its extensive settings dialogs to select a different new tab page.

• If you want to use your home page on the new tab: Try https://addons.mozilla.org/firefox/addon/new-tab-homepage/

• If you want to set a different page or a completely blank page: Try https://addons.mozilla.org/firefox/addon/new-tab-override/

Is this at all helpful?

please also refer to https://support.mozilla.org/en-US/kb/how-stop-firefox-making-automatic-connections#w_tiles_2

Thank you very much jscher2000 and philipp.

philipp, I believe modifying the "about:config" no longer mask the new tab page after the recent update.