I fixed this by hardcoding firefox as my browser on line 194 of
/usr/bin/webhttrack:

before: launch_browser "${BROWSEREXE}" "${SRVURL}"
after:  launch_browser "firefox" "${SRVURL}"

That script doesn't play nice with chrome, it assumes the call to launch the
browser will block, but it doesn't and everything exits, leaving the browser
with no htsserver to server the pages.