I don't think ja-JP-mac will be a proper standards locale (and http://mxr.mozilla.org/l10n-central/source/ja-JP-mac/ is just a repo name/location). See also https://bugzilla.mozilla.org/show_bug.cgi?id=418485#c9 . BTW, compatibility.js is not included in firefox/mozilla-central/b2g/chromuim builds