It launched first on iOS, where Firefox itself launched last. It had very limited features and a singled-minded focus on privacy. 1 million downloads already in that time, encouraging Mozilla to roll out even more futures for the simple browser.
Especially considering Google just boasted having over 2 billion monthly active devices. Downloads also don’t take into account daily active users. Firefox Focus, however, is more like a burner browser, that you use for one-off browsing activities and then throwaway afterwards, chucking along browsing cache and data.