Mozilla Technical Program Manager Chris Peterson accuses Google of making YouTube run slower on non-Chrome browsers. Peterson points out YouTube's latest design relies heavily on the deprecated Shadow DOM v0 API, which is only available in Chrome.
Firefox and Edge get served a Shadow DOM polyfill, which results in initial page load times of 5 seconds, versus just 1 second with Chrome. In a Twitter feed, Peterson offers some workarounds to improve performance.
The executive has also mentioned a couple of workarounds for users on Edge and Firefox, which involve the use of extensions to restore the pre-Polymer version of YouTube.
Peterson has also suggested that another way to fix the problem would be to offer the older version of YouTube to users on affected browsers, which is what Google does for Internet Explorer 11.
YouTube serves a Shadow DOM polyfill to Firefox and Edge that is, unsurprisingly, slower than Chrome's native implementation. On my laptop, initial page load takes 5 seconds with the polyfill vs 1 without. Subsequent page navigation perf is comparable.