Click to Play per-element demo for Scratchpad (devtools.chrome.enabled = true, Environment – Browser) or something similar. Based on code from https://addons.mozilla.org/firefox/addon/click-to-play-per-element/

ClickToPlay_per-element.js

let WindowListener = {
	handleEvent: function(aEvent) {
		var trg = aEvent.originalTarget;
		if( // I don't know, how do better checks here :(
			trg.className != "hoverBox"
			|| String.toLowerCase(trg.localName) != "div"
			|| !trg.parentNode
			|| trg.parentNode.className != "mainBox"
			//|| String.toLowerCase(aEvent.target.localName) != "embed"
			|| !(aEvent.target instanceof Ci.nsIObjectLoadingContent)
		)
			return;

		var window = aEvent.currentTarget;
		var document = window.document;

		let plugin = document.getBindingParent(trg);
		_log("plugin: " + plugin);
		let contentWindow = plugin.ownerDocument.defaultView.top;
		// gBrowser.getBrowserForDocument does not exist in the case where we
		// drag-and-dropped a tab from a window containing only that tab. In
		// that case, the window gets destroyed.
		let browser = window.gBrowser.getBrowserForDocument ?
			window.gBrowser.getBrowserForDocument(contentWindow.document) :
			null;
		// If browser is null here, we've been drag-and-dropped from another
		// window, and this is the wrong click handler.
		if (!browser) {
			//aEvent.target.removeEventListener("click", window.gPluginHandler._overlayClickListener, true);
			return;
		}
		let objLoadingContent = plugin.QueryInterface(Ci.nsIObjectLoadingContent);
		// Have to check that the target is not the link to update the plugin
		if (!(aEvent.originalTarget instanceof window.HTMLAnchorElement) &&
			(aEvent.originalTarget.getAttribute('anonid') != 'closeIcon') &&
			aEvent.button == 0 && aEvent.isTrusted) {
			if (objLoadingContent.pluginFallbackType ==
				Ci.nsIObjectLoadingContent.PLUGIN_VULNERABLE_UPDATABLE ||
				objLoadingContent.pluginFallbackType ==
				Ci.nsIObjectLoadingContent.PLUGIN_VULNERABLE_NO_UPDATE)
				window.gPluginHandler._showClickToPlayNotification(browser, true);
			else
				this.activateSinglePlugin(window, contentWindow, plugin);
			aEvent.stopPropagation();
			aEvent.preventDefault();
		}
	},
	activateSinglePlugin: function PH_activateSinglePlugin(window, aContentWindow, aPlugin) {
		_log("activateSinglePlugin()");
		let objLoadingContent = aPlugin.QueryInterface(Ci.nsIObjectLoadingContent);
		if (window.gPluginHandler.canActivatePlugin(objLoadingContent))
			objLoadingContent.playPlugin();

		let cwu = aContentWindow.QueryInterface(Ci.nsIInterfaceRequestor)
			.getInterface(Ci.nsIDOMWindowUtils);
		let pluginNeedsActivation = this._pluginNeedsActivationExceptThese(window, [aPlugin]);
		let browser = window.gBrowser.getBrowserForDocument(aContentWindow.document);
		let notification = window.PopupNotifications.getNotification("click-to-play-plugins", browser);
		if (notification) {
			notification.remove();
		}
		if (pluginNeedsActivation) {
			window.gPluginHandler._showClickToPlayNotification(browser);
		}
	},
	_pluginNeedsActivationExceptThese: function PH_pluginNeedsActivationExceptThese(window, aExceptThese) {
		let contentWindow = window.gBrowser.selectedBrowser.contentWindow;
		let cwu = contentWindow.QueryInterface(Ci.nsIInterfaceRequestor)
			.getInterface(Ci.nsIDOMWindowUtils);
		let pluginNeedsActivation = cwu.plugins.some(function (plugin) {
				let objLoadingContent = plugin.QueryInterface(Ci.nsIObjectLoadingContent);
				return (window.gPluginHandler.canActivatePlugin(objLoadingContent) &&
					aExceptThese.indexOf(plugin) < 0);
			});

		return pluginNeedsActivation;
	}
};

window.addEventListener("click", WindowListener, true);
setTimeout(function() {
	window.removeEventListener("click", WindowListener, true);
	_log("!!! Stop !!!");
}, 10e3);

function _log(s) {
	Components.classes["@mozilla.org/consoleservice;1"]
		.getService(Components.interfaces.nsIConsoleService)
		.logStringMessage("[c2p]: " + s);
}