[xfce] Can't open a file or folder directly from Firefox's "downloads" window

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http://ubuntuforums.org – Here's a strange little problem that's cropped up for me since I installed the XFCE desktop environment (I am running standard Ubuntu 12.04 but used the repositories to get XFCE) As is the default, I use Thunar as my file manager. I set Nautilus as preferred file manager briefly to see if it would fix the problem, but no luck. Anyway, here's the error message that pops up when I try to open a file or containing folder from the Firefox download window: Failed to launch preferred application for category "FileManager" Failed to execute child process "/usr/lib/x86_64-linux-gnu/x (Hardware)