Tons of extensions don't work. My personal blocker is mouse gestures; the webextensions API simply doesn't support the workflow I've been following for 15+ years. Mouse gestures will NEVER work well on Firefox 57+, just as they will NEVER work well on Chrome.
Additionally, mouse gestures are completely broken on both linux and MacOS due to Firefox's implementation of right-clicking and that is not slated to be fixed before v57 releases.
Oh-- and there's no functional LastPass addon yet either. And 57 is in beta 4 and will be released very soon.
It is pretty fast, though. Good for benchmarks if not for actually using. Personally, I switched to Vivaldi.
57 just entered Beta. It will be released on the 14th November. That's a whole 6 weeks still.
It also is not the end of all Firefox development. Just because it's not necessarily done for 57, doesn't mean that it won't get fixed or that it will "NEVER" work well.
It's also nonsense to make assumptions about what will work in Firefox in the future based on what works in Chrome, as Mozilla is introducing additional APIs that Chrome does not have.
If you're happy with Vivaldi, your thing. But there also is Firefox 52 ESR, which will continue to get security updates until the 26th June 2018.
Additionally, mouse gestures are completely broken on both linux and MacOS due to Firefox's implementation of right-clicking and that is not slated to be fixed before v57 releases.
Oh-- and there's no functional LastPass addon yet either. And 57 is in beta 4 and will be released very soon.
It is pretty fast, though. Good for benchmarks if not for actually using. Personally, I switched to Vivaldi.