Mozilla has claimed that Google's redesign of YouTube performs worse on the Firefox and Microsoft's Edge browser compared to Chrome. As per the accusation, YouTube's pages load five times slower in Firefox and Edge than in Chrome. The reason cited for the issue has been claimed to be YouTube's recent redesign that uses Google's Polymer technology. To recall, last year, YouTube had received a design refresh with Google's Polymer library that enabled "quicker feature development".
In a thread on Twitter, Mozilla's Technical Program Manager Chris Peterson has claimed that YouTube's Polymer redesign relies a lot on the deprecated Shadow DOM v0 API, which is only available in Chrome. This, he says, results in five times slower page load speeds on browsers such as Microsoft Edge and Mozilla Firefox. He writes, "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."
In several other posts, Peterson has mentioned a few workarounds for Edge and Firefox browser users. They essentially involve the use of extensions to restore the pre-Polymer version of YouTube. He also suggested that another way to fix the issue would be to offer the older version of YouTube to users on affected browsers that is what Google does for Internet Explorer 11.
Interestingly, Peterson says that Polymer's latest versions support both Shadow DOM v0 and v1 APIs, yet Google still uses Polymer 1.0 with the deprecated API. Notably, Google has not commented on these claims yet.
To force YouTube to go back to the old design and avoid the slow page load speeds on browsers other than Chrome, there are several options. Mozilla Firefox users can download a YouTube classic extension. Meanwhile, Safari and Edge users will have to use a custom Tampermonkey user script to get the classic design.
Edge users will have to download Tampermonkey for Edge from the Microsoft Store. After enabling Tampermonkey in the browser, download a user script to force YouTube to the classic mode. Safari users can also download Tampermonkey. Similar to the method before, download the same user script after enabling Tampermonkey in the browser.