Sorry to be stupid, but unless I'm missing something obvious it still isn't embedding the image. Either the Flarum software itself is trying to find the image (rather than JS in the browser editing window), or else Flarum is only linking and therefore trying to look for it during rendering. In any case it doesn't seem to want to embed an image from behind my firewall. Just a thought, could Firefox be stopping a script from running?
Example:
![](http://centos8.home/dokuwiki/doku.php?id=playground:playground)