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.
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.
I don't have a laptop, but will when the Qualcomm Snapdragon Windows10 machines land, and will probably use Edge exclusively there.
It's important to watch where incentives run. Google has the incentive to profit from you. Microsoft and Apple have the incentive to make sure your browser and device work as efficiently and well together as possible. Mozilla focuses on privacy as their main feature.
Which brings me to my other point (which I always keep hammering on in browser threads): why don't 3rd-party browser focus more on battery? If you're on Windows, Edge is king in battery use. On macOS, its Safari. On Linux, none (but then Linux doesn't have good power management in general). We live in a mobile-first world these days, why not focus browser development there?
Edit: I'm not dissing the Firefox or Chrome team their efforts, but almost all the news articles or blogs I see speak about Chrome or Firefox getting faster. And they do, by significant jumps. But when there are battery improvements, they are usually pretty middling (say 10-20m more battery life). So all I'm saying: why not flip the focus?