Firefox 41 browser.newtab.url preference is missing in about:config. Firefox 40 or versions prior to it, allowed users to replace the New Tab Page with a custom web page or site like Google.com or any, by changing ‘browser.newtab.url’ preference value in about:config to a desired URL (this is also one of the ways to get rid of the New Tab Page that has privacy concerning suggested tiles feature).
Which no longer the case from Firefox 41 onwards (which currently is in beta), Mozilla has removed this preference because it has been found abused by search hijackers.
‘Custom New Tab’ add-on for Firefox lets you load a desired URL in the New Tab Page
1. Download and install the add-on.
2. Visit add-ons manager, click on its ‘Options’, enter the URL in place of ‘about:newtab’, for which you want the new tab to open.
2. Click ‘OK’ after that, done, press Ctrl+T to open the new tab page to see it loading a site you’ve set above in the extension’s options.
This add-on has worked as advertised without any issues. There is also another add-on on AMO that lets you achieve the same : ‘New Tab Override (browser.newtab.url replacement)’, you can download it from here.
Since both offers same feature, you need to install only one extension.