I went and thested browsing amazon.de and it showed “insecured content” icon again on "CDs & Vinyl) category page and this time when I went to see FF’s page info and browsed trough images loaded on the page, this time this non-https image was listed there:
htxp://g-ecx.images-amazon.com/images/G/03/music/marc/SMSAmazon2.jpg
For some reason the image didn’t appear on the site itself though, but it showed the image file itself on the pgae information media list and it seemed simple little advert for a music band. However, Adblock doesn’t seem to include this image as blocked element on the site so I’m not sure what would cause it to not show.
EDIT: apparently the said image was one of the images hiden behind Amazon’s normally hidden browsing menus that appeared once I put my mouse on one of their category banners. Sorry about the commotion :-[