browse-url-firefox does not work with Firefox Quantum.
If Firefox Quantum is already running, then I am told:
Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.
If Firefox Quantum is not running yet, then the url is opened in the old Firefox version, which I apparently still have installed.
Does anyone know of a new implementation that does? Or knows enough about the new interface to implement it?