Well i have explored number of ways to open youtube as an anonymous user and as a user signing in their account.
I will be explaining the ways according to users skill level.
1. Beginners (proxy site)
2. Intermediate-expert (proxy tool)
If you want to add this as an "ADD-ON" in mozilla firefox, you will be required to drag and drop wj-button file to firefox toolbar.
For that , copy the URL (address from the address bar), close that window.Close all the tabs if anyone of them has a youtube page opened. Open the site www.unblockproxy.net and paste the copied URL in its bar.
New page will be opened, it might ask you for a human authentication check (entering alphabetical code from the screen). Enter that code. then video will be opened. After couple of seconds IDM download bar will appear as usual and this time around,you will be able to download.
IDM must be installed already on your computer and integrated to chrome,firefox (avoid internet explorer).