Firefox is my default browser anywhere but macOS, owning an iPhone and iPad I end up using the continuity features in Safari quite heavily when I've been browsing something on my phone or tablet and want to pick it up on my iMac.
I love Firefox, and owning an iMac I'm not terribly concerned about the power efficiency difference between the two - but damn if it isn't convenient to have the ability to just pick up where I left off.
There are reasonable answers -- Safari's extensions aren't quite as capable as the other two, so it's possible you're relying on an extension that Safari doesn't have and can never duplicate, and of course if you need to sync bookmarks and other browser data across OS platforms, Safari falls down in the way all iCloud-backed apps do. But Safari does focus a lot on battery conservation, and if you're on a laptop that can make a big difference. And, while I'm sure there are benchmarks out there to quibble over, in practice Safari seems to be just as fast and just as compliant as Chrome and Firefox are.
I've been using Safari as my main web browser for years without feeling like I'm missing out on either performance or killer features. I keep copies of Firefox and Chrome around and keep them up to date, mostly for web site testing purposes; neither of them have added features compelling enough to make me want to switch back. I doubt Firefox 57 is likely to change that equation, although I'm happy to hear that they've got their groove back, as it were.