Funny, I've done the opposite. Firefox has always been my default as far as I can remember but for the past several releases it has been heavy in terms of macOS memory. The author is obviously using Windows, but on macOS it is the opposite. Every startup, Firefox seems to throttle my MBP fans and will get sluggish as I keep on using it.
Same for me too (I'm using Windows), I've moved on to Chrome because of annoying performance issues with Firefox. Much better in that area after switching , although I still miss Tree Style Tabs sometimes... (you can't find a replacement for that in Chrome)