Type about:config<enter> in the address bar. If a warning screen comes up, press the Be Careful button. At the top of the screen is a search bar. Type browser.newtab.url This tells Firefox what to show when a new tab is opened. If you want, right click and select Modify. You can change the setting to;

about:home (Firefox default home page), about:newtab (shows the sites most visited), about:blank (a blank page),

or you can enter any web page or about: page you want.

The same instructions are used for the new window setting, listed as browser.startup.homepage.

browser.newtab.url doesn't work in Firefox 41 Beta - that preference was removed - it was being "abused" by hijacker malware.

You'll need something like this extension to add that feature back into Firefox 41 and later versions. https://addons.mozilla.org/en-US/firefox/addon/new-tab-override/