That code is from the 'remote handling' section. Remote handling is what happens if you have aMule running on a server and want to send an ed2k link to the server. You should use this instead for 'local handling':
REGEDIT4
[HKEY_CLASSES_ROOT\ed2k]
@="URL: ed2k Protocol"
"URL Protocol"=""
[HKEY_CLASSES_ROOT\ed2k\DefaultIcon]
@="C:\\Program Files\\aMule\\amulegui.exe"
[HKEY_CLASSES_ROOT\ed2k\shell]
@="open"
[HKEY_CLASSES_ROOT\ed2k\shell\open]
[HKEY_CLASSES_ROOT\ed2k\shell\open\command]
@="\"C:\\Program Files\\aMule\\ed2k \"%1\""
Just add it to your registry, it will overwrite the changes you made earlier. Then try again with Firefox.