Actually, I have. It gives me better tracking protection than vanilla Safari, it allows me to use SearXNG, and it means I can set a custom homepage. Also, for some reason, Safari has no private mode on my phone.
Not to mention that Mozilla is working on a Gecko-based version of the browser, as Apple is being forced by the EU to allow sideloading, third-party app stores, and third-party browser engines.
The big thing holding me back from Firefox on iPhone is the ad blocking side of things. Is there a way to block ads besides vpn based methods?
Well, there is NextDNS (and other providers, such as AdGuard DNS, rethinkDNS, and AhaDNS). It’s not a VPN, and it’s what I’m currently using alongside ProtonVPN (through WireGuard).
You could also try a Pi-Hole.
Good luck with it!
I have it but I have never really had much reason to use it over iphone + content blockers + DDG search.
I guess it’d be helpful for syncing bookmarks with desktops where I use firefox, but I don’t really worry about that, and as for passwords, I’ve got Bitwarden for that.