Here's a few:

- performance gains from optimized machine code

- CNAME adblocking, which is otherwise not possible on Chromium-based browsers

- robustness against potential changes from upstream (i.e. Manifest v3)

And, not exactly adblocking, but other privacy protections that are only possible with a deep level of control over browser APIs:

- advanced fingerprinting protections, including subtle randomized perturbations of canvas reads, fonts, user agent, etc.

- ephemeral storage sandboxing for third-party content