Hyper-Threading only allows programs to ‘see’ 16 hardware threads. It may or may not be a good thing.
It’s extremely difficult to spread CSS (in your example) across multiple cores. The point I was making was Mozilla are trying to move away from the monolithic approach (HTML, CSS, JS, etc. all on one core) to a more modular one (HTML on one core, CSS on another, etc.).
Regarding the “smaller parts == more power efficient” claim, define “efficient”. They use less power, yes, but that doesn’t make them more efficient. Power leakage is a major problem when making them smaller and so is manufacturing them if the fabrication process is new.
This is a Mozilla Research project and, as I’ve said before, this is only an experiment. It’s not going to replace anything yet but it might come in handy if this yields positive results.