Firefox is using the gtk-3.0 File Chooser Dialog and sometimes it would be nice to have some additional Directories in the Default/Favourite list on the left.
To change this, you just have to add the paths to the ~/.config/gtk-3.0/bookmarks file:
#~/.config/gtk-3.0/bookmarks file:///mnt/backup file:///path/to/directory
uriesk 