This option was once available, but it seems that is has disappeared in r250 (14.04). You might also want to delete the files in /home/phablet/.cache/webbrowser-app/thumbnails/
This option was once available, but it seems that is has disappeared in r250 (14.04). .cache/ webbrowser- app/thumbnails/
You might also want to delete the files in /home/phablet/