If I run Nightly with the command "./nightly/firefox-bin", and then click a link in an external application, the link opens correctly in the running instance of Firefox.
If I run Nightly with Wayland enabled with the command "env GDK_BACKEND=wayland ./nightly/firefox-bin", then clicking a link in an external application shows me the profile selection window, as if I had just launched Firefox.
If I run Nightly with the command "./nightly/ firefox- bin", and then click a link in an external application, the link opens correctly in the running instance of Firefox.
If I run Nightly with Wayland enabled with the command "env GDK_BACKEND=wayland ./nightly/ firefox- bin", then clicking a link in an external application shows me the profile selection window, as if I had just launched Firefox.
I'm running Nightly build 20181120100045.