Don't use copy/paste type downloaders, YouTube is fighting these programs (they don't seem to like it when we download a video).
Use "DownloadHelper" as it fakes YouTube into thinking you're watching the video, not downloading it.
There is also a nice converter that integrates into the program also.
Get DownloadHelper for Firefox HERE.The converter program is obtained by clicking the link in DownloadHelper after enabling conversion.
The instructions for all this can be read by going HERE.
Thank you for your reply, i will try that and see what happens.