They used to also use the unreleased version 0 of shadow DOM for building the Polymer UI, which - being a Chrome-only prototype - understandably didn't work on Firefox, and therefore instead used a really slow Javascript polyfill to render its UI.
I haven't checked on it lately, but I imagine they must've changed at least that by now.
One thing you can test is to apply a Chrome user-agent on Firefox when visiting YouTube. In my personal experience that actually noticeably improves the situation.
There's a bunch of extensions that allow you to switch user-agent easily, I personally use this one, it includes a list of known strings to choose between as well.
Google does that a lot with their own web properties. I remember Google Meet didn't support background replacement on Firefox, but switching Firefox's user agent to Chrome suddenly fixed it.
You haven't experienced slow until you try to take Firefox through Google Cloud Console or Search Tools. 15 seconds in Chrome, somehow turns into 3 minutes in Firefox, funny how it does that.
Ironically I use a chrome type browser for YouTube and mail checking only. This is also the only browser in which I am logged in with my Google account.
My main Firefox is for everything else including search.
That's a really weird take. Like… what even is the difference supposed to be?
This sounds more like “everything should be as it was back when <insert arbitrary point in time here>! When there were still Webpages, and we were frolicking about the internet! Until the fire nation attacked Web apps took over!”
In general, browser benchmarks seem to often favor Firefox in terms of startup and first interaction timings, and often favor Chrome when it comes to crunching large amounts of data through JavaScript.
I.e. for pages which use small amounts of JavaScript, but call into it quickly after loading, Firefox tends to come out on top. But for pages which load lots of JavaScript and then run it constantly, Chrome tends to come out on top.
We're usually talking milliseconds-level of difference here though. So if you're using a mobile browser or a low-power laptop, then the difference is often not measurable at all, unless the page is specifically optimized for one or the other.