I should google the design process but maybe they're doing it to make the input box match the padding of the drop down menu. I don't remember how it worked in FF74 (it's one of those things that I don't notice if they just work) but disabling the new behavior with browser.urlbar.update1 false makes the drop down menu expand to the full width of the window, so no problems with padding. The new menu is as narrow as the URL box and padded, so they had a problem.

I don't like moving interfaces (I disabled all the movable parts in Gnome Shell, extensions authors be blessed) so I'm sticking with browser.urlbar.update1 false until it goes away in FF 77.