There is no HTML file which one can edit to change the looping, this would have to be edited in source, and recompiled. Source Code. This is done by adding the line element->SetLoop(true);
around line 110. Function Definition.
If you'd prefer not to edit source, from https://support.mozilla.org/en-US/questions/993718, javascript:var vids=document.getElementsByTagName("video"); for (i=0; i<vids.length; i++) vids[i].setAttribute("loop", "true"); void 0;
can be saved as a bookmarklet to be run; or WhatIsThisImNotGoodWithComputers, on GitHub, has created a GreaseMonkey Script which automates the loop attribute setting.