hello unparadomas, try this in order to reset the new tab page manually: enter '''about:config''' into the firefox address bar (confirm the info message in case it shows up) & search for the preference named '''browser.newtab.url''' - right-click and reset that entry to the default value. you also might want to run a full scan of your system with different security tools like the [http://www.malwarebytes.org/products/malwarebytes_free free version of malwarebytes] and [http://www.bleepingcomputer.com/download/adwcleaner/ adwcleaner] to make sure that this change wasn't caused by adware or that it is present in other places of the system as well. [[Troubleshoot Firefox issues caused by malware]]

Some add-ons and even other programs you install may try to change your settings.