There’s an issue with Figma documents embedded in Confluence or any third party web pages.
The documents are not displayed in Firefox and the issues seems to be this:
“a website is trying to display another website without the consent of its owner. This is usually the result of a security misconfiguration.”
source: Website will not allow Firefox to display the page if another site has embedded it | Mozilla Support
This issue appeared at the start of 2023, so i’m assuming some new convention kicked into effect with an update. Chrome still displays the embeds, at least for now.
It is worth investigating and setting up the permissions properly if that is indeed the issue.