//
try {(nsvo => {
	var o = Cu.getGlobalForObject(nsvo).Object, {freeze} = o, NEW;
	o.freeze = obj => {
		if (Components.stack.caller.filename != "resource://gre/modules/AppConstants.jsm")
			return freeze(obj);
		obj.MOZ_REQUIRE_SIGNING = false;
		if ((NEW = "MOZ_ALLOW_ADDON_SIDELOAD" in obj))
			lockPref("extensions.experiments.enabled", true);
		else
			obj.MOZ_ALLOW_LEGACY_EXTENSIONS = true,
			lockPref("extensions.legacy.enabled", true);

		return (o.freeze = freeze)(obj);
	}
	lockPref("xpinstall.signatures.required", false);
	lockPref("extensions.langpacks.signatures.required", false);

	nsvo = Cu.import("resource://gre/modules/addons/XPIInstall.jsm", {});
	var shouldVerify = nsvo.shouldVerifySignedState;
	nsvo.shouldVerifySignedState = addon => !addon.id && shouldVerify(addon);

	if (NEW) nsvo.XPIDatabase.isDisabledLegacy = () => false;
})(
	"permitCPOWsInScope" in Cu
		? Cu.import("resource://gre/modules/WebRequestCommon.jsm", {}) : Cu
);}
catch(ex) {Cu.reportError(ex);}

(this.mcopy || (this.mcopy = (() => {
	var copy = win => win.navigator.clipboard
		.writeText(new XMLSerializer().serializeToString(win.document));
	var func = (copy, id) => copy(
		Cc["@mozilla.org/appshell/window-mediator;1"]
			.getService(Ci.nsIWindowMediator)
			.getOuterWindowWithId(id)
	);
	var url = "data:charset=utf-8," + encodeURIComponent(`(${func})(${copy}, `);
	return () => {
		if (gBrowser.selectedBrowser.isRemoteBrowser) {
			var pmm, {osPid, outerWindowId} = Services.focus
				.focusedContentBrowsingContext.currentWindowGlobal;
			for(var ind = 1, num = Services.ppmm.childCount; ind < num; ind++) {
				var child = Services.ppmm.getChildAt(ind);
				if (child.osPid == osPid) {pmm = child; break;}
			}
			return pmm?.loadProcessScript(url + outerWindowId + ");", false);
		}
		var data = {};
		Services.focus.getFocusedElementForWindow(content, true, data);
		copy(data.value);
	}
})()))();

try {CustomizableUI.createWidget({
	id: "ucf-view-source",
	label: "Код",
	tooltiptext: "Serialized document",
	localized: false,
	defaultArea: CustomizableUI.AREA_NAVBAR,
	onCreated(btn) {
		btn.creator = this;
		btn.setAttribute("oncommand", "creator.start(event)");
		btn.setAttribute("image", "data:image/svg+xml;charset=utf-8,<svg xmlns='http://www.w3.org/2000/svg' width='16' height='16'><g><rect x='0' y='0' width='16' height='16' rx='1' ry='1' style='fill:rgb(0, 120, 173);'/><path style='fill:white;' d='M 9.32,1 5.82,14.8 6.68,15 10.2,1.22 Z M 4.72,3.62 1,8 4.72,12.4 6.25,11.7 3.04,8 6.25,4.25 Z M 11.3,3.62 9.75,4.25 12.9,8 9.75,11.7 11.3,12.4 15,8 Z'/></g></svg>");
	},
	observe(s, topic) {
		Services.ppmm.removeMessageListener(this.msgName, this);
		Services.obs.removeObserver(this, topic);
	},
	addTab(win, src, type) {
		var url = `view-source:data:${type};charset=utf-8,${encodeURIComponent(src)}`;
		var gb = win.gBrowser, pos = gb.selectedTab._tPos;
		gb.moveTabTo(gb.selectedTab = gb.addTrustedTab(url), pos + 1);
	},
	receiveMessage(msg) {
		this.addTab(Services.wm.getMostRecentWindow("navigator:browser"), ...msg.data);
	},
	start(e) {
		Services.ppmm.addMessageListener(this.msgName = "UCF:VS:SerializedDocArrived", this);
		Services.obs.addObserver(this, "quit-application-granted", false);

		var func = (serialize, id) => {
			var win = Cc["@mozilla.org/appshell/window-mediator;1"]
				.getService(Ci.nsIWindowMediator).getOuterWindowWithId(id);
			win && sendAsyncMessage("NAME", serialize(win));
		}
		var serialize = (win, doc = win.document) => [
			new win.XMLSerializer().serializeToString(doc),
			win.HTMLHtmlElement.isInstance(doc.documentElement)
				? "text/html" : doc.contentType
		];
		var url = "data:charset=utf-8," + encodeURIComponent(
			`(${func})(${serialize}, `.replace("NAME", this.msgName)
		);
		(this.start = e => {
			var win = e.view, br = win.gBrowser.selectedBrowser;
			if (br.isRemoteBrowser) {
				var pmm, {osPid, outerWindowId} = Services.focus
					.focusedContentBrowsingContext.currentWindowGlobal;
				for(var ind = 1, num = Services.ppmm.childCount; ind < num; ind++) {
					var child = Services.ppmm.getChildAt(ind);
					if (child.osPid == osPid) {pmm = child; break;}
				}
				return pmm?.loadProcessScript(url + outerWindowId + ");", false);
			}
			var data = {};
			Services.focus.getFocusedElementForWindow(br.contentWindow, true, data);
			this.addTab(win, ...serialize(data.value));
		})(e);
	}
});} catch(ex) {Cu.reportError(ex);}

@-moz-document url-prefix(chrome://custombuttons/content/editor.xul) {
	link[href="chrome://global/skin/dialog.css"] + vbox + hbox.dialog-button-box {
		background-color: red !important;
	}
}

...
		var [ss] = document.documentElement.shadowRoot.styleSheets;
		ss.insertRule(
			`.dialog-button-box {
				margin-bottom: 4px !important;
				padding: 3px !important;
				margin-top: 22px !important;
			}`,
			ss.cssRules.length
		);