This effect has been mentioned many times on HN. Some comments attribute it to Google intentionally slowing Firefox to make Chrome seem faster. Some comments

This effect has been mentioned many times on HN. Some comments attribute it to Google intentionally slowing Firefox to make Chrome seem faster. Some comments attribute it to Google utilizing API's in Chrome that they have specifically optimized to make gmail faster that are not so "specially optimized" in Firefox. In the end, only the Google chrome and gmail teams can answer the true why, everything else is guesswork on everyone's part. As to youtube, as with @PaulHoule) I too have not noticed any slowness, but my use of youtube in firefox is only to obtain the URL to hand off to yt-dlp (youtube-dl successor). Watching videos with my local client player is worlds superior to playing a stream in any browser (Firefox/Chrome/otherwise) that I simply do not play the streams in Firefox from youtube at all.