Team Mozilla is working on a new design for Firefox 57, which is currently on nightly branch. Along with the new logo, this version may come with tonnes of new features which include faster page loading, smoother scrolling, and more responsive tab switching.
The new logo which might be released with version 57 shows a slight tilt in the tail of the fox along with an altogether new colour theme.
This version is expected to have modern and intuitive design, Photon UI along with suggestions in the address bar and ‘one click search’ with which you can send query directly to a single site.
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Though this version is on nightly branch but you can still download it for testing purposes from HERE.
If you find any bug, you can anytime report it to Mozilla HERE.
You can also have a look at Experimental Features in Firefox.