maybe I just have to make a userstyle instead :/

popoutvideo.js

// ==UserScript==
// @name        Lesschrome popup of HTML5 video
// @description   Lesschrome popup window of the current url; ensure you uncheck "Always show the tab bar" in Firefox tab settings
// @include     *
// @version     1
// @grant         none
// ==/UserScript==

function obj_to_settingsstr (obj) {
	var settingsstr = "";
	for (var key in obj) {
		if (obj.hasOwnProperty(key)) {
			settingsstr += key+"="+obj[key]+",";
		}
	}
	return settingsstr.replace(/,$/,"");
}

function add_button (video) {
	var button = document.createElement("input");
	button.type = "submit";
	button.value = "pop out";

	var settings = {
		resizable: "yes",
		width: video.videoWidth,
		height: video.videoHeight,
		// Pretty much none of the below works – how can I get
		// a "clean" pop-out video??
		scrollbars: "no",
		status: "no",
		location: "no",
		titlebar: "no",
		menubar: "no",
		toolbar: "no",
		personalbar: "no"
	};

	button.onclick = function() {
		var lesschrome_window = window.open(
			window.location,
			"already_lesschrome",
			obj_to_settingsstr(settings));
		lesschrome_window.onload = function() {
			// Move the already playing video into the
			// lesschrome window so we don't start over
			// again:
			video.parentElement.removeChild(video);
			lesschrome_window.document.body.replaceChild(
				video,
				lesschrome_window.document.body.firstChild);
			video.play();
		};
		button.onclick = function () {
			window.location = window.location;
		};
		button.value = "reload";
	};

	video.parentElement.appendChild(button);
}

if(window.name !== "already_lesschrome"
   &&
   "tagName" in document.body.firstChild
   &&
   document.body.firstChild.tagName === "VIDEO") {
	add_button(document.body.firstChild);
}