Not the original commenter, but Firefox has noticeably poorer performance on my MacBook pro 2018, especially on react-heavy sites like the AWS Console or Twitch.tv
Mostly this, performance is atrocious on a lot of sites I use.
I also dislike its history management, download management, auto-complete, search in address bar functionality, pocket integration, and much more. I'm sure if I were forced to spend more time with it I could possibly find configurations to customize all those things in a way I like, but even using it for a few days the easily findable settings weren't flexible enough.
I realize your comment says you’ve already switched back, but should you ever try Firefox again most of these can be tweaked directly from about:config
Firefox has awful performance on nearly any web page for me (2014 Mac). Even its own settings page alone puts a core at 100%. I can get past bad font rendering and giving up pinch-to-zoom and not reading PDFs in the browser, but I can't give Firefox half my battery life. I'll use it on my work PC but it's still a huge waste of electricity.
I also dislike its history management, download management, auto-complete, search in address bar functionality, pocket integration, and much more. I'm sure if I were forced to spend more time with it I could possibly find configurations to customize all those things in a way I like, but even using it for a few days the easily findable settings weren't flexible enough.