fix: Fix Firefox Developer Edition config
fix: Fix Firefox Developer Edition config

1. Firefox Developer folder name in OS X is FirefoxDeveloperEdition.app, not
   FirefoxDeveloper.app
2. Existing Aurora users were auto-updated to Firefox Developer Edition without
   changing the folder where the browser was installed; this commit makes the
   configuration fallback to the previous directory.

karma-runner/karma-firefox-launcher,oyiptong/karma-firefox-launcher,karma-runner/karma-firefox-launcher,jamestalmage/karma-firefox-launcher