The latest Firefox update introduced a new new tab page, which shows thumbnails of your most recently and frequently visited web sites, which you can rearrange, delete, or pin. You can even add bookmarks from the bookmarks sidebar, toolbar or menu.

But if this is not for, you can also add a custom page to be loaded whenever you ask for a new tab page.

To customize it,

1. Go to advanced preferences entering about:config in the location bar
2. Search for preference browser.newtab.url
3. Double-click on it and enter the web address (URL) of the page you want to load (like a custom portal, your Facebook profile, etc.), or to set a blank page, enter about:blank.

From now on, when you open a new tab, your favorite page will be loaded.