Works fine for me:
$ xprop -root _DT_SAVE_MODE
_DT_SAVE_MODE(STRING) = "xfce4"

Reporter, what does
xprop -root _DT_SAVE_MODE
report for you then?

and what version of xdg-utils are you using?

I suppose we could fallback to try $DESKTOP_SESSION env, on my xfce-4.6 environment, it reports

$ echo $DESKTOP_SESSION
xfce4

(and apparently xfce 4.8 returns just 'xfce', but I'd like to confirm if you see that too)

xprop -root _DT_SAVE_MODE is used by xdg-open script to detect that currently running DE is xfce. For me it worked fine in xfce 4.6, but in 4.8 it doesn't work. 

$ echo $DESKTOP_SESSION
xfce

(not xfce4)

I'm using xdg-utils 1.0.2 in Arch

Sorry, forgot to paste this:

$ xprop -root _DT_SAVE_MODE
_DT_SAVE_MODE:  no such atom on any window.

Confirming this problem. I use Archlinux as the reporter. People, it's really serious, Chromium opens all files in Mozilla Firefox (!!) and Xarchiver does the same. I had to manually edit all xdg-* scripts (can't send a patch because it's a dirty hack :)

I'd suggestion someone with contact with xfce upstream... ask them how best to detect it.

mkkot, as a workaround I would suggest to make "export DE=xfce" somewhere. This act as a fallback entry for xdg-open when desktop environment isn't detected.

Bug #25941 seems to be related.

(In reply to comment #9)
> mkkot, as a workaround I would suggest to make "export DE=xfce" somewhere. This
> act as a fallback entry for xdg-open when desktop environment isn't detected.

This doesn't work since this commit: http://cgit.freedesktop.org/xdg/xdg-utils/commit/?id=c9551d250722081cbf7d6611f417bc597765e5c9 .

In retrospect, I'm unsure of the wisdom of clearing DE in that referenced commit.  Perhaps that portion should be reverted (unless there was some good reason for it, that I fail to fathom).

(In reply to comment #11)
> In retrospect, I'm unsure of the wisdom of clearing DE in that referenced
> commit.  Perhaps that portion should be reverted (unless there was some good
> reason for it, that I fail to fathom).

Totally agree, I had to remove this line so that my workaround with "export DE=xfce" could work again.

Commenting out this DE clearing helped me. It doesn't overwrite my "export DE=xfce" in .xinitrc. Ideally there should be a check if it was already defined manually.

this should hopefully do the trick:

http://cgit.freedesktop.org/xdg/xdg-utils/commit/?id=a9d65ff51ce0dd40cf0cb39b0f3c36dca100c527

For posterity, I would still appreciate someone investigate why/how xprop fails on some platforms and/or ping xfce upstream for a better or proper way to detect it's use.