Serenity still has Ladybird as it is right now. So I assume that will become the baseline for "Browser" in SerenityOS going forward, and be developed independently. Whatever the browser in SerenityOS will end up looking like though, I doubt it will see much development. The kinds of people interested in working on browsers will just work on Ladybird.

Someone will probably once again port Ladybird to SerenityOS in the future. It won't be part of the main system, but users would be free to install it.