The final picture of Photon’s design is so close in Firefox 57, that the latest Nightly update has added flexible spaces around the URL and Search bar by default and the bookmarks button on the navbar has been replaced by Library button. The bookmarks button has moved inside the library menu.

Mozilla has added spacers to the toolbar to keep location bar away from buttons Home and reload buttons, and also to center the location bar within the browser window.

This has been done by Mozilla, according to the Photon design spec. Both address and search bar looks truncated now, Of course, you can make Firefox navbar look like before, for that, open customize dialog and remove the spaces.

Firefox Nightly : Stylo enabled by default for Some 

Firefox 57 to offer Migration UI in Activity Stream to import other browser data 

Firefox 57 Nightly: Photon Rectangular Tabs, New Icons and more 

Firefox Photon new Overflow menu and Customize mode 

Firefox 57 Photon Onboarding Tour