Header element gets ignored when pasted in the editor

[azkarim]

Editor : http://neilj.github.io/Squire/
Sample header : https://azkarim.netlify.app/scrap/header.html

Copy and pasting the sample header into the editor results in nothing being appeared in the editor.
Note this is only applicable if you are visiting the sample header site from Firefox

Chrome and Safari changes it to p tag.
You can check the underlying markup by pasting here https://html-online.com/editor/