The nightly builds of Firefox feature a revamped address bar design

In nightly builds Firefox, based on which the release of Firefox 71 will be formed on December 2, activated the new address bar design. The most noticeable change is the shift from displaying a recommendation list across the full width of the screen to transforming the address bar into a distinctly defined window. To disable the new appearance of the address bar, an option 'browser.urlbar.megabar' has been added in about:config.

Megabar continues to develop a new implementation of the Quantum Bar (browser.urlbar.quantumbar), which first appeared in Firefox 68 and features a complete rewrite of the code, replacing XUL/XBL with standard Web API. In the first phase, the Quantum Bar design fully mimicked the old address bar, with changes limited to internal restructuring. Now, work has begun to put into practice previously stated ideas for modernizing the appearance.

Source: opennet.ru