> Httrack works wonderfully, but when I use Webhttrack
> it fails.  This is what I get when I try to run it
> from the command line:

Well, apparently, /usr/bin/x-www-browser links to the run-mozilla.sh script,
which does not seems to be able to execute /opt/firefox/x-www-browser-bin

> and more importantly, how do I fix it?
You can try the following, which *should* work:
rm -f /opt/firefox/x-www-browser-bin
ln -s firefox-bin /opt/firefox/x-www-browser-bin

(Apparently there's a problem with this release of Ubuntu  and
update-alternatives)