Assign mailto: for Firefox to launch new mail window for evolution, add this line for Mailer:
evolution-1.5 mailto:%A?Subject=%S&Cc=%C&body=%B
Er, why mangle the mailto: URL you were given? That's just bizarre.
Stick to 'evolution %r' and you get the rest of the headers preserved too. Some mailing list archives give mailto links with In-Reply-To: headers intact, etc. and your version would gratuitously discard them.