We had this issue where the H5P iframe flickers in Firefox but working fine in other browers.

We found out that firefox sometimes returns the value 1 for element clientHeight. in this case h5p-resizer.js functions prepareResize and resize change iFrame height to different values making the iFrame flickers.

We solved it by changing

iframe.style.height = data.clientHeight + 'px'; 

by

iframe.style.height = (data.clientHeight > data.scrollHeight ? data.clientHeight : data.scrollHeight) + 'px';

Regards,