That’s because “firefox” (or “chrome” too) on apple products is just a reskin of Safari. Apple does not allow 3rd party browser engines in its app store.
That’s because 3rd party browser engines might not suck ass, which would allow OWA apps in your browser whcih would circumvent Apple’s 30% cut on everything. So they kneecap their own browser and don’t allow any other browser on their devices.