Fix invalid path for Firefox user.js file on macOS

The path for the custom Firefox `user.js` file (1) file was invalid for
macOS hosts. It doesn't contained the `Profiles` directory causing a
symlink to a path where the file won't be read by Firefox.

References:
  (1) http://kb.mozillazine.org/User.js_file

Fixes GH-175

snowblocks/firefox/snowblock.json

@@ -35,7 +35,7 @@
           "iceowl": "profiles.iceowl.ini"
         }
       },
-      "~/Library/Application Support/Firefox/igloo/user.js": {
+      "~/Library/Application Support/Firefox/Profiles/igloo/user.js": {
         "create": true,
         "force": true,
         "hosts": {