What are the features missing then? The main one I can see is edit PDFs directly[0], but I'd bet money that Mozilla wouldn't implement that on iOS anyways.
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.
Or is it not "can't use Firefox on iOS" but "doesn't have x feature from Firefox on Desktop"?