I traced the problem to this file: /usr/share/xfce4/helpers/firefox.desktop
When I changed the existing line:
X-XFCE-CommandsWithParameter=%B -remote "openURL(%s)";%B %s;
To this (adding quotes to %s):
X-XFCE-CommandsWithParameter=%B -remote "openURL(%s)";%B "%s";
the problem went away.
I traced the problem to this file: /usr/share/ xfce4/helpers/ firefox. desktop
When I changed the existing line:
X-XFCE- CommandsWithPar ameter= %B -remote "openURL(%s)";%B %s;
To this (adding quotes to %s):
X-XFCE- CommandsWithPar ameter= %B -remote "openURL(%s)";%B "%s";
the problem went away.