Firefox buildchain does not fixate `rust` and `cbindgen` versions,
so we want to fixate them on our end.
A table with matching rust version for every firefox version can
be found at [Rust Update Policy for Firefox](https://wiki.mozilla.org/Rust_Update_Policy_for_Firefox).
Additionally, there are checks in `mozbuild` for the minimum
rust version and minimum `cbindgen` version.
This strictly defines Firefox build folder as `obj-build-playwright`.
Currently, Firefox build folder encodes current Mac OS version including
patch versions, and thus we might end up with multiple different build
folders.
