Can Browsers on iOS use Content Blocking? Can any of the third party browsers use the content blockers that are available in Safari? Is this an API limitation?
For example none of the browsers like Firefox or DuckDuckGo apper to use the filters from AdGuard.
I believe not, unfortunately. If you ask BlockBear (the add filter I personally use):

Nope. Apple limits the content blocker to Safari at the moment, only Mr. Cook knows the future.