Good Old Youtube also does a brilliant job of re-instating the non-polymer youtube interface (which is more lenient towards computer resources, especially applicable on older hardware/OSes), but, as it is an id-less Web Extension not supported on Serpent 55/52, it doesn't install out-of-the-box ; but if you modify its manifest.json file to include a gecko-id, then it will install (and work) OK!
{
"name": "Good Old YouTube",
"manifest_version": 2,
"version": "1.15.0.1-unsigned",
"description": "Switch back to classic YouTube interface!",
"icons": {
"96": "images/logo.svg"