you have to edit the file /usr/bin/webhttrack
and modify the following line
#SRCHBROWSEREXE="x-www-browser www-browser iceape mozilla firefox iceweasel
abrowser firebird galeon konqueror opera netscape"
# modified
SRCHBROWSEREXE="iceape mozilla firefox iceweasel abrowser firebird galeon
konqueror opera netscape"
the problem is that on some ubuntu based distros x-www-browser is not set and
www-browser is setup to w3m

you can confirm if that is your case:
update-alternatives --get-selections | grep browser

which means that webhttrack will try to use w3m by default, by modifying the
webhttrack startup script you should be able to start it in gui browser.