For Chrome:
1. Go to the following link and install the extension by clicking the install button
http://tinyurl.com/6cj5ekt
2. Select the following check boxes and deselect all the rest. You can now browse Youtube with no ads before the video's!

For Firefox:
1. Go to the following link and install the plugin by clicking add to firefox. http://tinyurl.com/6ejvyu4

2. Restart Firefox, subscribe to the default profile that they present you with and browse youtube with no ads before the video's!