For a long time, I thought Firefox was just slow. After all, it was my main browser, and I have more than a hundred tabs open at any given time. I found that when scrolling around some sites (including this very one - my own), performance would come to a halt. I do not have a powerful computer: it's just an i3 from several generations ago. I often saw Firefox taking up 30% CPU, so I thought horrible scrolling performance was a result of Firefox using so much CPU. I didn't think scrolling was the cause of so much CPU usage - because my site scrolled fine in Chrome!
At work, I also use Firefox as my main browser. I also have hundreds of tabs open there. Firefox CPU usage hovers around 4%. I chalked it up to the powerful 4th gen i7 CPU in my workstation.