Firefox being slow has almost nothing to do with Mozilla’s incompetence or the browser’s inability to handle websites.
When devs build websites, they usually build them for the most popular browser, aka Chrome. They couldn’t be bothered to help the minority of people who use Firefox. Also, cost. Building a website to work with 2 different engines is more expensive than building it for just one engine that’ll work for 99% of users. That’s why a lot of banking websites never support FF.
Another primary reason is Google’s Monopoly. Almost everyone uses some Google service or another. Google’s websites are tailored to perfectly fit Chromium, not FF. This is why you’ll sometimes see websites break or even crash. YouTube’s recent ambient mode made the site choke quite a lot on FF. An average Joe ain’t got the knowledge to know or even troubleshoot the issue and they’ll just shift to Chromium, where everything just works.