"The only downside"
Sure. Aside from the number of other extensions that simply won't work in new Firefox. And the lack of any user control to make the browser not work like or look like Chrome.
How is that? The omnibox is toggle-able. The entire browser UI is drag-n-drop customizable. I literally don't know of a single thing that is less customizable in 57 than 56, but I'm curious to hear more about what you're referring to?
And downvoted for asking someone to explain their handwaving false criticisms. Hope this place never changes.