It's not an issue of having the resources to implement the spec -- it's that they're not convinced of the value of the feature, especially given it has the potential to impact performance.
I personally think they are vastly underestimating the enormous potential upside for the whole Internet if devs can make style changes with confidence, and frameworks like Vue and Svelte don't have to keep bolting on bespoke, expensive JavaScript-based approximations.
That's an interesting way of saying "Firefox is not backed by an unlimited pile of cash"