The best advice I can give is: don’t immediately abandon the stock apps. I see tons of people who get an iPhone, immediately install Chrome and Gmail and all I can think is, what’s the point?
AdGuard or a similar adblocker for Safari will give you the results you got from Firefox. Safari on iOS also supports full desktop extensions if the developer chooses to make them available. So things like 1Password work great on it.
Same with the Mail app, Calendars, etc. Try setting up your accounts and services in the stock apps and see if you like them. Besides that, there isn’t much to tweak, that’s kind of the point.
If you want app recommendations, it depends on what services you use. Some of my favourite apps are:
Weather - CARROT
Mastodon - Ivory
Lemmy - Bean or Voyager
Package Tracking - Parcel
RSS - Reeder
Password Manager - 1Password
Remote Management - Remotix
Home Server Management (Sonarr/Radarr/SabNZBd) - LunaSea