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"
},
"background": {
"scripts": [
"shared.js",
"options/default-options.js",
"logger.js",
"reconstruct.js",
"background.js"
]
},
"content_scripts": [
{
"matches": [
"https://www.youtube.com/*"
],
"css": [
"styles/hide-alert.css"
],
"js": [
"shared.js",
"content-script.js"
],
"run_at": "document_start"
}
],
"options_ui": {
"page": "options/options.html"
},
"permissions": [
"https://www.youtube.com/*",
"storage",
"webRequest",
"webRequestBlocking"
],
"applications": {
"gecko": {
"id": "{482060de-6804-4020-a1b9-16dc012a3c93}"
}
}
}
(The id string is identical to the one generated when the extension is installed in FirefoxESR 52.9.x)