My Xilinx ise installation has library trouble.
The programs in /opt/Xilinx/13.3/ISE_DS/ISE/bin/lin64/ can't find it's libraries in /opt/Xilinx/13.3/ISE_DS/common/lib/lin64/. I can put the directory in ld.so.conf but then it screws up firefox and other apps. So I put a little script

LD_LIBRARY_PATH="/opt/Xilinx/13.3/ISE_DS/common/lib/lin64/" <program> but it doesn't find them.

What's the missing magic, pls?

First of all this is a commercially licensed product so http://www.xilinx.com/support/ should be your first port of call IMHO. That said you haven't indicated you have satisfied all dependencies and you haven't posted output (strace?) that would help us help you.

I take your point about it being commercial software. That said, I do not have a commercial license, but use the (free) webpack part as a student. The support for that seems to be a faq and a forum, where users can talk to each other.

I know the issue from this support forum: It is a library version issue - they use old versions of libstdc++ libraries (and presumably other things). People have downgraded gcc with mixed success to try and fix it. I never bothered with strace, as ldd /path/to/program |grep found showed up the problem, and the fix.

I was hoping I had made a mess of the syntax, to feed it itś own libs, but I gather I had not. What annoys me is that firefox barfs with the xilinx directories at the bottom of /etc/ld.so.conf. Firefox already has a mention (higher up) in ld.so.conf, as that is not comfortable with standard versions either.

No, syntax looks OK. See if it has settings that increase verbosity or enable debug mode?