I'm using Nightly (v58a) and you can customize the UI a lot more than in the current release (v56).
You can move back/forward and refresh, auto-hide certain buttons like downloads, add items to the "overflow" menu, toggle full/compact mode, toggle title bar, toggle a "grab" space...there are a lot of options.
They've also added proper support for light and dark icons for plugins. So now buttons like 1Password don't stand out with a dark theme.
To me it feels like a return to the level of customization I'd gotten used to in older versions of Firefox. And FWIW, this is what mine looks like: https://i.imgur.com/seJZeg6.png
What you just described is literally nothing like the level of low-level customization that past releases of the Mozilla Suite/Firefox had to offer. Not only can extensions no longer modify any of the browser XUL/chrome://, you can't even install Complete Themes that previous let you control the layout of every single browser css element.
It's like Mozilla is deliberately committing corporate suicide with these hasty feature removals.
Is it now possible to make the title bar blue when the window is active and grey when it's not? The lack of this feature (that once used to be the norm) is one of the most painful things for me.
You can move back/forward and refresh, auto-hide certain buttons like downloads, add items to the "overflow" menu, toggle full/compact mode, toggle title bar, toggle a "grab" space...there are a lot of options.
They've also added proper support for light and dark icons for plugins. So now buttons like 1Password don't stand out with a dark theme.
To me it feels like a return to the level of customization I'd gotten used to in older versions of Firefox. And FWIW, this is what mine looks like: https://i.imgur.com/seJZeg6.png