It is? The problem is that the authoritative source sits inside the Firefox tree. Mozilla makes a standalone version (that doesn't require that tree) available

It is? The problem is that the authoritative source sits inside the Firefox tree. Mozilla makes a standalone version (that doesn't require that tree) available on GitHub, but you're not going to have much luck contributing back if your contributions break the Firefox build. The solution proposed would be to make the GitHub the authoritative source, and patch the Firefox version. But that shifts the maintenance burden in the way that's opposite to the main developers' interests. I can see why there's no enthusiasm. I've contributed to libcubeb and had no problems with the above. But my contributions were not of the "lol I don't like automake so let me replace your entire buildsystem" kind. Honestly if that's the kind of thing you're trying to do, I think almost any open source project will push back.