Firefox has no extensions so no Adblock.
That's because so far every browser on iOS had to use WebKit as it's HTML rendering engine, meaning that even if you installed another browser manually you were basically still using Safari under the hood. IIRC the new DMA rules include allowing other browser engines like Gecko, so Mozilla is probably already working on making addons available. I mean they are available on Android, so why wouldn't they make them available on iOS now that they finally can?
You have this backwards. Google showing you captchas is basically them saying they can't match your browser to any know (shadow) profile they have already stored. So they aren't sure you are a human and if so which one specifically. Getting harassed with a captcha is essentially like a badge of honour for your browsers privacy settings.