1. In the Location bar, type about:config and press Enter. The “This might void your warranty!” warning page may appear.
2. Click I’ll be careful, I promise! to continue to the about:config page.
3. Type browser.newtab.url in the search box to find it on the list.
4. Double-click on browser.newtab.url and type in about:newtab in the window that pops up.

Restart Firefox and open a new tab to make sure the change was made