This started happening a little while ago, so I should not be using these for Tutorials since many people use Firefox, and cannot see the hidden content.
<details> <summary>Click here to reveal hidden content</summary> If you can see this, then it is working in your Browser. </details>
Test it here by clicking this:
Click here to reveal hidden contentIf you can see this, then it is working in your Browser.
Please check if there is a fix for this on Discourse.
EDIT: here is a post on Discourse regarding this problem in FF:
Works fine in Chrome …