Well looks like the final days for classic youtube... you'll be missed my friend.
Google has announced that it will kill off the classic YouTube desktop interface in March 2020. This script will probably stop working!
Update 1.0.10: Remove the "classic youtube is going away" banner.
This script was created because my youtube scripts (Youtube Play Next Queue and Youtube Search While Watching Video) originally didn't work with the new youtube redesign. But you can certainly also use this for other scripts that only work with the old classic youtube.
If you want to use modern youtube again: