gnome-shell-browser-plugin does not work with Firefox 60 ESR

Issue

• The "gnome-shell-browser-plugin" does not load in Firefox60 ESR. As a result, "extension.gnome.com" functionality does not work in the browser anymore.

• Going to "https://extensions.gnome.org/local/" lists no extensions on Firefox 60, but does on Firefox 52.

• Browsing to an extension shows no "ON/OFF" toggle on Firefox 60, but does so on Firefox 52.