This can be done using socat:
socat TCP-LISTEN:5000,fork TCP:localhost:9000
This redirects all tcp connections from port 5000 to 9000. To run this command, the 5000 port shouldn't be already in use.
Use this addon on firefox.
YouTube\'s new design uses Polymer 2.0, and the userscript below no longer works. Since as of right now this question is the top Google hit for queries related to changing this behavior I have created a Chrome extension and a Firefox 57+ add-on which I open sourced in case anyone wants to look at the internals (or create a port for other browsers). Please note that I will no longer be maintaining the userscript version.
Source: stackexchange