According to The Register, privacy-conscious users have sounded the alarm after it emerged the “New Tab” thumbnail feature in Firefox 13 is “taking snapshots of the user’s HTTPS session content”.
Firefox actually introduced the thumbnail capturing capability in Firefox v12 and did not tell the users; however, there is no way to display them in v12. Firefox v13 displays the thumbnails when a new tab is selected.
NetAnalysis v1.54 can extract Mozilla Firefox Thumbnail Images
We added the ability to extract these thumbnail images (stored in the cache) to NetAnalysis v1.54. See the following for further information on moz-page-thumb entries.