There does not seem to be a way to process a custom .Xmodmap file with the /usr/bin/xmodmap command at every startup of xfce. I put the .Xmodmap file in the home directory, and it works perfectly when xmodmap ~/.Xmodmap is being called from the xfce terminal after xfce startup. However, NONE of the following produces the desired keybindings: - adding /usr/bin/xmodmap /home/userA/.Xmodmap to my ~/.xinitrc when starting with startx, adding it to my ~/.config/xfce4/xinitrc, to the autostart items via Sessions and Startup, or via hand-coded .desktop files in the autostart folder and combining with all possible combinations of home folder specification. It seems that some internal process overrides the settings or prevents them from being activated. I can exlude that they are executed twice, as I do not only swap keys.
*** This bug has been marked as a duplicate of bug 5094 ***
maybe I should have added that it also doesnt work if I choose to use the system defaults. It seemed to me that 5094 referred to the case when system defaults are overriden.
In that case there must be something wrong with your setup, as it works perfectly for me, and quite a few others. I do recall there was a weird bug in some version of xinit that would cause this to fail. You can try upgrading that package if possible.
Hm, I'd like to have tracked it down further, but have already deinstalled xfce and switched to awesome, where it works fine (as it does in lxde). Thanks anyway for looking into it.