I believe Apple's officially endorsed method of adding extension support to browsers built on `WKWebView ` is via [BrowserEngineKit](https://developer.apple.com/documentation/browserenginekit).
Bug 1497374 Comment 2 Edit History
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I believe Apple's official method of adding extension support to browsers built on WKWebView is via BrowserEngineKit. https://developer.apple.com/documentation/browserenginekit