Easy download youtube video with IDM and any format to u should like .flv .mp4 etc....
it's comfort with Firefox
Step 1: Download Bestvideodownloader and install in your PC.
Step 2: Restart Firefox
Step 3: Open youtube and choose any video you want.
Step 4: Click on Download.
Step 5: Select type to download
Step 6: Open IDM and Paste to AddURL.
Enjoy :)