I started to use the prebuild version for Firefox and I really enjoy the new GUI so far. It's much cleaner looking and really nice to navigate in.
Since it's a prebuild I'm pretty sure you already know that it contains bugs and errors but I thought that I could post the ones I found anyway.

The issue I've seen so far is:
When you press the "star" icon in a login field to show your saved logins, the window that opens up is cut-out so you can't really see much of the content inside.
It seems to occur if you only have one login to the site, if u have multiple logins it looks much better and you can actually see all of the content.
I posted 2 pictures to show you how it looks:
This is multiple logins where you can see your logins properly:

This is a single login where everything is cut-out:

Hope you can fix this in the future, keep up the good work!