I have read this thread: https://support.mozilla.org/en-US/questions/934935 but the solution this user found apparently isn't available to me. I cleared my cache and cookies the other day, and now my New Tab Page isn't populating with new thumbnails. I do have one thumbnail, a page I pinned, but all of the others haven't come back. I went into about:config and found browser.newtab.url, but the 'reset' option was greyed out. I haven't been able to find out what the next step is to restore my thumbnails; am hoping someone here can help. Thanks!