Earlier quoted context omitted.
https://apps.apple.com/us/app/firefox-private-safe-browser/i... ? Or is it not "can't use Firefox on iOS" but "doesn't have x feature from Firefox on Desktop"?
Last time I checked, it was just a Safari webview, and missed a lot of features compared to Firefox on Android. Has that changed recently? Other nice thing of Firefox on Android: being able to listen to Youtube videos even when the screen is turned off. (I'm sure there are solutions, but many seemed sketchy or didn't work well)
FF can largely do exactly the same thing they do on Desktop to attract people: create a privacy-focused browser (if that's even what they're doing) and things like syncing your bookmarks, passwords, using Firefox VPN, and blocking ads.