Thank for your work, your plugin is very great.
After made some test I see that when you using firefox the Fetching of thumbnail is not gracefull like in IE or Chrome.
You can see that there :
When you see Pinterest RSS Widget area, just refresh your browser (F5).
For me is not a theme or WP version issue, because I see this problem in my both web site that running different theme and WP version.
It's a small issue, but it will be great if there is a fix.
Thanks for your helps