My Debian desktop has the following alternatives setup:

www-browser (/usr/bin/w3m)

mozilla (
 +    1        /usr/bin/mozilla-suite
*     2        /usr/bin/mozilla-firefox
)

x-www-browser (
      1        /usr/bin/mozilla
      2        /usr/bin/epiphany
 +    3        /usr/bin/konqueror
*     4        /usr/bin/mozilla-firefox
)
[download]

So either mozilla or x-www-browser seem like sensible "launch-url" commands.