What did we do:

First we installed the latest "xulrunner" and "xulrunner-devel" via yum (v1.9.2.4-9.el5)

The we copied the contents of the newly installed xulrunner to the xulrunner-dir within zdesktop. We didn't clear the xulrunner-dir within zdesktop, cause there's a lot more in it than just xulrunner

Now it looks like it's working like it does on the Window$ platform

So just in CLI:

#yum install xulrunner-devel -y
#cd /usr/lib/xulrunner-1.9.2
#cp -r * /opt/zimbra/zdesktop/linux/prism/xulrunner/ --reply=yes
No guaranty that it works , but now at least I can add my account(s)
My ZDesktop is synchronizing now