I don't know why, but even on my machine which gets 40-60 FPS in FFXIV while simultaneously encoding a movie, Firefox was always slower than chromium browsers.
And of course: wgich extensions do you have in firefox and how old is your profile. Try it out with new, clean profile and than you will feel it.
I mean this is fair, but eventually the profile ages and I may choose to add more extensions, no? Why would a selling point be "we're fast on a brand new install, but after a couple years and adding some extensions, we're gonna slow down like fuck"?