I have been using Firefox for basically as long as I can remember and I love it. However, there's one website that I go to Chromium for: GeoGuessr/Google Street View. For some reason it's unbelievably slow and sluggish in Firefox whereas it works normally in Chromium. Why could this be? To be clear, it's only the Street View part (and moving/panning/zooming) that's slow on GeoGuessr.
Do you mean the time when YouTube's UI was built using a pre-standardized version of the Shadow DOM API, and had to polyfill it in Firefox? If so, that was tech debt, not artificially slowing down page loads for Firefox on purpose. It was a tradeoff that let non-Chrome users use YouTube until they finally upgraded a year or two later.
If that's not it, I'd love to see what you're referring to.