> Also, qbrt doesn’t yet support runtime version management (i.e. being able to specify which version of Gecko to use, and to switch between them). At the time you install it, it downloads the latest nightly build of Firefox.

This doesn't sound so good, and is the opposite of what I think people have come to expect. It's seems like the most straightforward solution is to hardcode a specific version of the runtime into the package, so that installing qbrt XX.0.0 gets you the runtime that corresponds to Firefox version XX.