I said it for Waterfox and I'm gonna say it again for Firefox: this is good. At worst, it's just fine (Mozilla just uses it internally to replace or supplement its old and incomplete Tracker Blocking system, which never gets the same scrutiny).
The biggest difference between Firefox and Waterfox in implementation is the WaterFox developers noticed this FF change early, and committed to providing full-fledged ad blocking out of the box, which is great news for users.
A few more reasons this is good:
- Rust is faster than JavaScript
- Native functionality is faster than an extension
- Actual ad blocking is something Firefox users have been begging Mozilla to do