Short answer: add the Firefox folder you want to add to your gtk-bookmarks file. It will then appear in the firefox file chooser in the left hand pane above the Other Locations thingy.
Create or edit ~/.gtk-bookmarks and add the full file URL to there:
Code:
file:///home/yourusername/Firefox
That works for me. Make sure the folder has been unveil'd first.
Hope that helps
Simon