I've read, and re-read, this post several times. I'm pretty sure I understand the problem correctly, yet trying to search for a solution is quite difficult considering the wording involved (i.e. desktop, open, problem, trouble, linux) - you can imagine the search results.
I keep thinking along the lines of filetype associations or mime-types. I have yet to produce anything useful to the problem at hand related to those two _possibilities_. Another guess is that the problem lies in xfce, perhaps the desktop handling components of xfce (e.g. xfwm or some such thing). Maybe a component of xfce is broken??? Maybe it's user-specific.
Have you tried creating a new user (via vasm) and then logging into the new user account to see if the problem persists? Might be worth a shot.
Good luck and if I think of anything more I'll post here.