I gave it a spin about 4 months ago and stuck with it for about 2 months and was still having the issue. A Mozilla employee pointed me here to watch for updates on the issue:
I still have severe performance issues with Firefox on my Mac also. When I mentioned it a couple of weeks ago in a post here I was downvoted and basically accused on making it up.
https://bugzilla.mozilla.org/show_bug.cgi?id=1404042
Edit: Also relevant: https://github.com/servo/webrender/issues/3115