youtube-opml-exporter-script
Greasemonkey firefox addon script restoring RSS exporter functionality for youtube, which has been recently removed
Youtube has recently removed the subscription_manager page
it now redirects to
where there is no button to export your subscription list as an OMPL file, i.e.
https://www.youtube.com/subscription_manager?action_takeout=1
Just install Greasemonkey in Firefox, create a new script and copy & paste the code found here.
It will add two download buttons (top & bottom) to the page with which you can do an opml export!
This script could potentially be used by Chrome via its Tampermonkey addon, but I have not tested it.