I've used Firefox for a full, busy day at work and I've loved it. There are a few things that feel odd, but I'm sure I'll get used to them. I especially love the 'Scratchpad'. It's super handy!
One thing that might be a dealbreaker is the seemingly lack-of Service Worker developer tools.
I'm certainly going to give Firefox with Duck Duck Go as my search engine a proper run. It feels good to get one more step away from Google. Especially now I no longer use Gmail.