Firefox Focus appears under Safari settings > content blockers, along with more typical blockers like AdGuard. I think they both implement Content Blocker API.
The way Firefox Focus is implemented on iOS is a little complicated, since it's both a browser and a Content Blocker. As of 2017, Firefox Focus uses WKWebView as the webview component: https://github.com/mozilla-mobile/focus-ios/pull/507
WKWebView does not support the Content Blocker API. Ad blocking apps that use the Content Blocker API are only compatible with Safari and the SFSafariViewController component, which is very feature-limited and not suitable for a full web browser app:
As a workaround, Firefox Focus uses script injection to block ads and trackers within the browser part of itself, but the Content Blocker part of Firefox Focus only affects Safari:
However, both Firefox Focus and Safari (with the Firefox Focus Content Blocker enabled) score 78% on that test, so Firefox Focus might be good enough for web browsing on iOS if you're comfortable with its feature set and don't need the additional filter lists or custom rules that another third-party Content Blocker would offer.
Apple should still allow third-party browsers to use third-party Content Blockers, since this restriction is an unnecessary handicap for any non-Safari browser on iOS.
WKWebView does not support the Content Blocker API. Ad blocking apps that use the Content Blocker API are only compatible with Safari and the SFSafariViewController component, which is very feature-limited and not suitable for a full web browser app:
- https://www.wwdcnotes.com/notes/wwdc20/10188/
- https://developer.apple.com/news/?id=trjs0tcd
As a workaround, Firefox Focus uses script injection to block ads and trackers within the browser part of itself, but the Content Blocker part of Firefox Focus only affects Safari:
- https://github.com/mozilla-mobile/focus-ios/blob/main/Blockz...
- https://github.com/mozilla-mobile/focus-ios/blob/main/Blockz...
- https://github.com/mozilla-mobile/focus-ios/issues/1761
However, both Firefox Focus and Safari (with the Firefox Focus Content Blocker enabled) score 78% on that test, so Firefox Focus might be good enough for web browsing on iOS if you're comfortable with its feature set and don't need the additional filter lists or custom rules that another third-party Content Blocker would offer.
Apple should still allow third-party browsers to use third-party Content Blockers, since this restriction is an unnecessary handicap for any non-Safari browser on iOS.