fyi: https://www.neowin.net/news/mozilla-executive-claims-that-google-has-made-youtube-slower-on-edge-and-firefox

"Early last year, YouTube received a design refresh with Google's own Polymer library which enabled "quicker feature development" for the platform. Now, a Mozilla executive is claiming that Google has made YouTube slower on Edge and Firefox by using this framework.

In a thread on Twitter, Mozilla's Technical Program Manager has stated that YouTube's Polymer redesign relies heavily on the deprecated Shadow DOM v0 API, which is only available in Chrome. This in turn makes the site around five times slower on competing browsers such as Microsoft Edge and Mozila Firefox."

Hope it helps.. ;)