Thread can be read on this article.

YouTube page load is 5x slower in Firefox and Edge than in Chrome because YouTube’s Polymer redesign relies on the deprecated Shadow DOM v0 API only implemented in Chrome. You can restore YouTube’s faster pre-Polymer design with this Firefox extension: https://t.co/F5uEn3iMLR

— Chris Peterson (@cpeterso) July 24, 2018

According to that article,

Google Chrome ads started appearing next to Firefox search terms. Gmail & [Google] Docs started to experience selective performance issues and bugs on Firefox. Demo sites would falsely block Firefox as ‘incompatible’

while Firefox was still a Google search partner.

EDIT: Did not realize how long ago this post was made, whoops.