In the latest release of Firefox, new tab pages display your nine most frequently visited Web pages in an easy-to-see thumbnail layout. You can customize the sites that are displayed and even hide the new tab page, but if you prefer disabling the feature permanently, here's how:
Step 1: In the Firefox address bar, type about:config and hit Enter.
Step 2: When the warranty warning appears, click on the "I'll be careful, I promise!" button.
Step 3: Type browser.newtab.url in the search box.
Step 4: Double-click on the preference name, then change the string value from about:newtab to about:blank. Click OK to save.
That's it. Now when you bring up a new tab in Firefox, it'll be blank, rather than displaying the thumbnails of your most frequently visited sites.