I just noticed that the itch.io game widgets on my website aren't working, and Firefox blames itch.io.  Exact error message:

Firefox Can’t Open This Page

To protect your security, itch.io 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.

Context: https://eldwood.com/a-dream-of-burning-sand/

I've got the same issue with my browser (Duckduckgo and Mozilla, also replicated on edge). 

I'm not sure what's causing it but it's not something we can fix in our side except switching to other browser. 

Thanks for your post, I was thinking I was doing something wrong (first time trying the widget).

Thank you for you post ! I had the same issue today using Firefox...

Same.
Well, if this new issue follows the same path as the animated GIF bug, that doesn't look too good...