AFAIK there does not seem to be a way to donate to rust development for the same reasons.

Maybe Mozilla should allow donors to specify the project they'd like to contribute to?

That would give them additional feedback on how the community evaluates their programmatic direction.

Part of the reason for that, I think, is that Firefox development is done by the Corporation, rather than the Foundation. The Corporation's can have far bigger income streams (i.e. the search engine deal), so it makes sense to make that responsible for the development of Firefox - even though that means you cannot donate to it.