This commit makes Firefox look for the default profile directory in a
directory relative to the binary path.
The directory can be specified through the --with-relative-data-dir.
This is relative to the same directory as the firefox main binary for
Linux and Windows.

On macOS, we remove Contents/MacOS from it.
Or, in other words, the directory is relative to the application
bundle.

This behavior can be overriden at runtime, by placing a file called
system-install adjacent to the firefox main binary (also on macOS).