Here are some easy steps to download video from YouTube from you Firefox web browser.
1) Right click the video which you desire to download and click on 'Copy Link Location'
2) Open a new tab and browse http://www.keepvid.com
3) Paste the link you just copied in the URL field and click on download
4) Shortly you will see some (3GP, FLV, MP4...) files on the page. Click on the desired file you wish to download.
That's all!!! Thankyou :)