Allow Firefox binary on macOS system path to take precedent over defaults. (#4)
Allow Firefox binary on macOS system path to take precedent over defaults. (#4)

This allows macOS to be consistent with Linux, where a Firefox binary discovered on the system path will now take precedent over the default installation paths. Previously, the system path would only be checked if there was no binary present in the default installation paths.
jgraham/rust_mozrunner