No Firefox on iOS, hence useless.
My impression is that the Firefox shell offered is still able to provide the various anti-tracking privacy features that many would point to Firefox for, and the variety of browser shells available should mean that you'd be able to find a UI to your liking if Safari's isn't.
At that point, the only thing I can see missing is a non-webkit engine. I get that that's an annoyance and definitely on the same anti-competitive level as 00s era IE, but by and large web developers account for it and it works acceptably. As much as I'd need it to for mobile browsing.
Would just be interested to know if there's something more I'm missing.
No plugins/add-ons effectively.
As a vague counter point, I use Firefox Focus[0][1] which touts the tracker blocking and ad blocking I'd rely on extensions for normally. It meets my needs as the only additional extensions I use on desktop are for tab and session cookie management, both of which are moot points in a browser without tabs and a "clear cookies after each session" policy.
[0] https://play.google.com/store/apps/details?id=org.mozilla.fo... [1] https://apps.apple.com/gb/app/firefox-focus-privacy-browser/...
Of course, nowadays the assets of apps have to be part of the deployable, itself. So it's common to run localhost web server.