When trying to open an URL with Firefox (eg. via File Manager of Mail Reader), Firefox opens a blank page or your homepage (if it's set to something other than blank) instead of the URL.
This situation has been reported and confirmed as a bug (bug #1427144 on launchpad).
It seems that the problem is in the file
/usr/share/xfce4/helpers/firefox.desktop, and replacing the last line fixes it.
You can fix it manually (1) or just run the command (2) that creates a backup and automatically replaces the broken line.
/usr/share/xfce4/helpers/firefox.desktopwith your favorite text editor and root permissions (eg.
sudo mousepad /usr/share/xfce4/helpers/firefox.desktop). Then replace the following line:
X-XFCE-CommandsWithParameter=%B -remote "openURL(%s)";%B %s;
With this one:
X-XFCE-CommandsWithParameter=%B %s;%B %s;
Automatic: Open a terminal window and run:
cd /usr/share/xfce4/helpers/ && sudo cp firefox.desktop firefox.desktop.bak && sudo sed -i 's/-remote "openURL(%s)"/%s/' firefox.desktop