*UPDATE* July 2017 - Mac Users on OS X Sierra running Safari 10 will not be able to select Folders from their local drive when using the advanced uploader.
*UPDATE* March 2017 - Newer versions of the Firefox browser (Versions 52 and newer) doesn't support Java apps such as the Flight Advanced Uploader.
There are two browser options for using Advanced Uploader on Mac: Firefox and Safari. On Windows: Internet Explorer and Firefox.
Advanced Uploader will not work with Chrome, as Chrome doesn’t support JSF.
If using Firefox,
On Windows: Use the 32-bit Firefox ESR browser. Make certain it is the 32-bit and not 64-bit. OR, Firefox version 51 still supports Java plugins.
On Mac: Firefox ESR supports Java plugins OR switch to Firefox version 51.
More information can be found here: https://support.mozilla.org/en-US/kb/npapi-plugins?cache=no