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.

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

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

No--question was how to stop websites from being added to tab page automatically. Answer does not address this.

The page is designed to reflect your history (frequently/recently used sites); I don't think there's a way to disconnect that feature. For any of the boxes that you don't assign to particular sites (either by pinning or by dragging in a bookmark), Firefox will use history. If you want those additional boxes to be "blocked" until used, you could assign a placeholder page for the time being.

Here's another thought: if you don't need 9 boxes, you can reduce the size of the grid to two rows of three thumbnails (6 boxes) or 2 by 2 (4 boxes) or whatever you like. To do that:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtabpage.columns preference and enter your preferred value.

(4) Double-click the browser.newtabpage.rows preference and enter your preferred value.