Hi, you can change the number of rows by clicking the gear wheel button in the top right corner of the page. You can also pin your own sites which will stop new ones being added automatically, by pointing your mouse at the top right corner of each thumbnail; in Edit, you can also add your own sites if you wish. More Customize your Firefox New Tab page.

Some Sync articles for reference: https://support.mozilla.org/en-US/products/firefox/sync

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I think that you will have to pin websites to tiles that you do not want to use for the history.

Pinned tabs are stored in the browser.newtabpage.pinned pref on the about:config page. The value of this pref is an array of a url and possible title value pair in JSON format. Tabs that aren't pinned have an null entry as you can see if you inspect this pref on the about:config page.

• [{"url":"<url>"},null,null,{"url":"<url>"}]

A way to specify blank tiles:

• [null,null,null,null,null,null,null,null,null,{"url":"about:blank?1"},{"url":"about:blank?2"},{"url":"about:blank?3"},{"url":"about:blank?4"},{"url":"about:blank?5"},{"url":"about:blank?6"},{"url":"about:blank?7"},{"url":"about:blank?8"},{"url":"about:blank?9"}]