See https://www.userchrome.org/megabar-styling-firefox-address-b... for details on how to customize the new address bar, including userChrome css for keeping its size confined to the dropdown UI like other dropdown/combobox UI elements.
This issue is frankly far more important than the style changes they made. It's a muscle-memory issue that drastically affects daily usage. I was so desperate to restore the sane behavior of not selecting everything on a single click that I was recompiling firefox prior to discovering this workaround.