Web browser ed2k protocol handling do not work on fresh install

Binary package hint: firefox

On fresh install, ed2k protocol is not handled by firefox.
What i tried :
In about:config added :
network.protocol-handler.app.ed2k /usr/bin/ed2k
network.protocol-handler.expose.ed2k false
network.protocol-handler.external.ed2k true
network.protocol-handler.warn-external.ed2k true

In Edit-> Préférences -> Applications :
ed2k is absent

In a terminal :
gconftool-2 -s /desktop/gnome/url-handlers/ed2k/command '/usr/bin/ed2k %s' --type String
gconftool-2 -s /desktop/gnome/url-handlers/ed2k/enabled --type Boolean true

amule-utils wich provides ed2k executable is installed and the program is present in /usr/bin

On another PC which was upgrades from maverick, there is no problem. On the machine where it works, it was configured with FF 3.6, the update to 4.0 from ppa mozilla-stable, then with FF 4.0 from natty upgrade.

On this machine, this is a fresh install.

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: firefox 4.0.1+build1+nobinonly-0ubuntu0.11.04.3
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic-pae 2.6.38.2
Uname: Linux 2.6.38-8-generic-pae i686
Architecture: i386
Date: Sat Jun 4 18:36:00 2011
FirefoxPackages:
 firefox 4.0.1+build1+nobinonly-0ubuntu0.11.04.3
 flashplugin-installer 10.3.181.14ubuntu0.11.04.1
 adobe-flashplugin N/A
 icedtea-plugin 1.1~20110420-0ubuntu1
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
ProcEnviron:
 LANGUAGE=fr_FR:en
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: firefox
UpgradeStatus: No upgrade log present (probably fresh install)