I'm wondering why this is not a simple setting on firefox. Are they trying to collect data on how many people have interest in privacy or something ? (another comment pointed that Firefox focus tracks users and reports to mozilla)
That was my initial reaction too, but I think it makes sense.
Everyone has use for both full-featured browsing where you can stay logged in to frequent sites, retain history/bookmarks and such, as well as private browsing where you don't want any history kept. On the desktop the way the UI is currently setup, full-featured is the default and private is an extra step you have to take, and as such private is typically only used for special occasions.
Focus reverses this dynamic by making private browsing easier, faster, and more streamlined than full-featured browsing, so people will start making private browser the default for most quick web tasks, and just go to the slower the full-featured browser when they need it.
Having two different launcher icons for these two modes is more convenient for the user, and AFAIK it isn't possible for a single app to install multiple icons in iOS[1]. Besides, not everyone wants to use Firefox as their main mobile browser. So a separate app makes sense.
