macOS can open links in new tabs when set as default browser (despite remoting being disabled)
Haven't had a chance to debug too deeply, but the best guess is that the relevant code path is here:
This code should likely be checking for the MOZ_NO_REMOTE env variable or something at that entry point and bailing early if it is present. I'm 90% sure this is an upstream bug so we should uplift once we have a patch.