"Most visited webpages" are calculated by Firefox. Other than repeatedly visiting a webpage there is no way for the user to influence that "calculation".

Change your Homepage to the New Tab page? about:newtab See - How to set the home page

Thanks a lot regarding the home page.

I've visited YouTube a million time but it still doesn't appear there. I will keep trying I guess. Thanks.

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 or tab and drop them in a tile on the about:newtab page to pin this item and have your own favorites sites showing.

See this article about the New Tab page (about:newtab):

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

Changes made manually on the about:newtab page are stored in prefs that you can find on the about:config page.

• browser.newtabpage.pinned stores websites that you have pinned to a tile.
• browser.newtabpage.blocked stores websites that you have removed by clicking the X button in a tile.

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