My understanding is that the "deliberate slowing" reported, was such that if you have firefox send a chrome user agent, that one gets faster (and functional) results.

Is that consistent with the "it is because FF doesn't support new thing yet" explanation?

I believe it was that YT was using the legacy v0 shadow DOM API that only Chrom[e|ium] supported, with a slow polyfill for other browsers, rather than the much better supported v1 API that all other major browser makers agreed to implement.