Hi Bo,
On my machine, all the programs (mozilla-firefox, webhttrack) are located in
the same directory (usr/bin). The webhttrack script already has usr/bin in the
search path.  I tried altering the search path to include the actual file
names but that didn't help. Here is the error I get when I try to run
webhttrack from the terminal:

bob@ubuntu:~$ webhttrack
/usr/bin/webhttrack(9127): launching /usr/bin/x-www-browser
/usr/bin/webhttrack(9127): spawning regular browser..

run-mozilla.sh: Cannot execute /opt/firefox/x-www-browser-bin.

/usr/bin/webhttrack(9127): waiting for browser to terminate..


I looked at the various other scripts that might be involved:
/usr/bin/mozilla, /usr/bin/mozilla-firefox, /usr/bin/mozilla-firefox.ubuntu,
/usr/bin/x-www-browser

but I can't see anything obviously wrong.  Do you have any other ideas about
what I can try next?
TIA,
Bob