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.
It's web builders deliberately building their sites and webapps for chromium based browsers only, because it has over 80% market share. They only test on firefox rudimentally. The experience is subpar and people use chromium instead because of it, cementing chromium as the most used browser. Some site builders do this because they don't have the time to extensively test a browser with low market share, others, like Google, do it deliberately.
UMatrix is no longer under development, right? uBlock has that functionality built in iirc. It came from the same dev, unless I'm confusing them with something else.
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"?
Blink is somewhat faster than Gecko in most sites, but it use somewhat more resources, because render every tab independly. Because of this some Chromium hibernate tabs in background (Chrome itself don't)