I too switched for several months, but am already back to using Chrome. Even with the performance improvements, it's still noticeably slower than Chrome.
I'm curious: what OS do you use and what sites are you visiting?
I've had a stellar experience on Arch Linux. The RAM usage is down. I can't say I really ever found Chrome slow, but nor is Firefox for me. I think they're both plenty fast, but I prefer sites without lots of garbage flying around on the page anyway.
macOS High Sierra. I generally have a lot of tabs open: two Inbox tabs, Facebook, StackOverflow chat + question pages for what I'm working on, many JIRA tickets, HN, sometimes Twitch, etc.
I think it has something to do with video rendering since opening Twitch/Youtube often causes the problems to start.