How can i made clicking on a link kmail message open firefox 3 instead of firefox 2.x

you can change the default www browser in system settings

you can also try to change names of /usr/bin/firefox - this is how it looks t my laptop:

#!/bin/sh

# Firefox launcher containing a Profile migration helper for
# temporary profiles used during alpha and beta phases.

# Authors:
# Alexander Sack <email address hidden>
# Fabien Tassin <email address hidden>
# License: GPLv2 or later

# if there exists a beta profile (first found: $MOZDIR/firefox-3.0,
# $MOZDIR/firefox-trunk, $MOZDIR/granparadiso) and there is a standard
# firefox profile as well, ask the user what to do. In case he decides to
# import the firefox 2 profile settings, we keep the firefox directory
# untouched, but rename the beta profile by appending the suffix
# |.abandoned|. In case the user decides to keep using the firefox 3
# profile, we will rename the original firefox profile to firefox.2-replaced
# and rename the beta profile to be |.mozilla/firefox|.
#
# as a third option the user can defer his final decision. This will leave the
# directories untouched, thus making the user use the old firefox profile by
# default.
#
# in addition, even older profiles are renamed too, by appending |.abandoned|
# to their name.

APPNAME=firefox
MOZDIR=$HOME/.mozilla
LIBDIR=/usr/lib/firefox-3.0b5
DROPPED=abandoned

FOUND=""
if [ -d $MOZDIR/firefox ] ; then
  FOUND=firefox
fi

FOUND_BETA=""
BETA_LIST=""
for betaname in granparadiso firefox-trunk firefox-3.0; do
  if [ -d $MOZDIR/$betaname ]; then
    BETA_LIST="$BETA_LIST $betaname"
    FOUND_BETA=$betaname
  fi
done

if [ "$FOUND" != "" -a "$FOUND_BETA" != "" ] ; then
  echo -n "Found Beta Participation ..."
  $LIBDIR/ffox-3-beta-profile-migration-dialog
  result=$?
  if [ $result = 1 ]; then
     mv $MOZDIR/$FOUND $MOZDIR/$FOUND.2-replaced
     mv $MOZDIR/$FOUND_BETA $MOZDIR/$FOUND
     for beta in $BETA_LIST ; do
       if [ $beta != $FOUND_BETA ] ; then
         mv $MOZDIR/$beta $MOZDIR/$beta.$DROPPED
       fi
     done
     echo " keep beta profile."
  elif [ $result = 2 ]; then
     for beta in $BETA_LIST ; do
       mv $MOZDIR/$beta $MOZDIR/$beta.$DROPPED
     done
     echo " use firefox 2 profile."
  else
     echo " use firefox 2 profile, but will ask again next time."
  fi
  echo " ... will check again next time."
elif [ "$FOUND_BETA" != "" -a "$FOUND" = "" ]; then
  # in case we only have a beta profile the user most likely wants to use
  # that. just doing, no questions asked.
  mv $MOZDIR/$FOUND_BETA $MOZDIR/firefox
  for beta in $BETA_LIST ; do
    # Move out the older beta profiles
    if [ $beta != $FOUND_BETA ] ; then
      mv $MOZDIR/$beta $MOZDIR/$beta.$DROPPED
    fi
  done
  echo "*NOTICE* Profile $FOUND_BETA found and moved as main profile"
fi

exec $LIBDIR/$APPNAME "$@"

this way every link is opened in ff3