Reports have surfaced suggesting security concerns with Firefox's new thumbnail display for recently-visited sites. Fortunately, it's fairly easy to work around.
As The Register notes, thumbnail images can display information such as email subject lines or banking details which you might not want to see after closing a tab. Mozilla is working to fix the issue in a future release, but if you don't want to wait, you can disable the option.
Computing weblog GHacks notes that you can access the configuration settings by typing about:config in your browser's address bar, finding the preference named browser.newtabpage.enabled, and then changing the value from true to false. This will disable the thumbnail new tab page and instead display a blank page.
Ghacks has two other ways to get around this security issue; hit the full post for more details.
How to turn off Firefox's New Tab Page Completely [GHacks Technology News]