You probably would have encountered Youtube videos starting automatically. I found out some cool Firefox extension that to disable these videos from playing automatically.

TubeStop Firefox Extension

The TubeStop firefox extension disables the autoplay feature of YouTube videos, as well as videos on other sites like MySpace and Blogger.

Download TubeStop Extension

Youtube Prevent Autoplay Greasemonkey Script

Most of us love Greasemonkey because it doesn’t bloats your pc memory that much then the Firefox extensions does.This is where Prevent Autoplay Script comes to the play, simple script that prevents youtube auto playing the videos.
Download Youtube Prevent Autoplay Script from userscripts.org

Method 3: Using the Stop Autoplay Extension

Stop Autoplay Extension is a one stop solution. After installing this extension, you’ll notice that the video is completely gone from the page. When you click on the video, then only it will start loading and playing.
Install Stop Autoplay Extension from Mozilla Add-ons

Method 4: FlashBlock Extension

You can also use the FlashBlock extension that completely block flash, it blocks other flash stuffs like banners, ads, applications etc, my advice use it only when you totally dont want any flashy stuffs.

Download FlashBlock Extension from mozdev.org