Maybe you changed the options.

• See New Tab page in Firefox
  • Customize your Firefox New Tab page

Other possibilities include some addon interfering, or maybe malware. Does ity help to try Firefox's safemode ?

• Diagnose Firefox issues using Troubleshoot Mode
• Troubleshoot Firefox issues caused by malware

You can look at the browser.newtab.url pref on the about:config page and set that pref to the URL of your preferred new tab page.

• the default new tab page with the tiles is about:newtab
• the default home page is about:home
• for a blank page you can use about:blank

The browser.newtabpage.enabled pref should be set to true to use the page set via browser.newtab.url as the new tab page.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config