I know from trying to use screen-size simulators to see how my storefront looks on different devices that Zazzle made behind-the-scenes changes quite a while back that disallow embedding.
To protect your security, www.zazzle.com will not allow Firefox to display the page if another site has embedded it. To see this page, you need to open it in a new window ... Learn More.
The "Learn More" takes you to this info page:
Website will not allow Firefox to display the page if another site has embedded it
I've never tried it in another browser but imagine that if they don't allow embedding, they don't allow it period so browser used won't make a difference.
