YouTube videos start streaming automatically. However, there might be occasions when you want to disable this – if opening multiple videos, or on a slow internet connection. There is no way to disable auto play within YouTube, and going by the number of complaints in various forums (including the official Google one), it certainly is sought-after.
Below are browser extensions and scripts to prevent autoplay of videos. Note that none of these prevent auto-buffering of videos (ie. loading videos).
For Google Chrome
Use Stop Autoplay for YouTube extension. It stops the auto play without killing the buffering, so you can put several background tabs to the task of buffering while watching other videos. This extension also works with the new and updated YouTube player (HTML5 player).
Tubestop Firefox addon blocks off automatic video streaming of embedded media, on YouTube and other websites.
Get Tube stop addon
YousableTubeFix is a Greasemonkey script (userscript) for Opera to stop auto loading of videos.