re-implement desktop onboarding for ESR 68

As of Firefox 64, the onboarding extension which we used to implement Tor Browser onboarding has been removed. See: https://bugzilla.mozilla.org/show_bug.cgi?id=1462415 https://bugzilla.mozilla.org/show_bug.cgi?id=1457565

More research is required, but it looks like Firefox's new onboarding experience is integrated into their "activity stream" interface (aka new tab page).