This change will break the capability of running multiple builds in a row. We want to remove that, but we shouldn't start a regression here.

So when you test your patch please see that you cover the following use cases:

The `binary` option specifies:
* the real firefox binary (or .app on OS X)
* an installer
* a folder which contains installers, which will be used sequentially
* a folder which does not contain the firefox binary