Gecko (and many other large C++ programs that care about memory usage of certain heap allocated types) uses bitfields often enough, and we've had to teach bindgen how to generate bitfields so that we can access them from Rust.
Were these intended as part of an external interface for Gecko? In which case I would say using bitfields (as opposed to explicit masks) was a mistake.
But yes, I can imagine that for Mozilla there is a need to mix Rust and C++ right inside an application at boundaries that are not usually considered external, in which case all kinds of non-portable C constructs are acceptable.
