Thunderbird is more for newsgroups...you could just use regular e-mail and put all of the addy's in the CC field, or the normal Sender field for that matter.
It would be better to put the email addresses in the BCC (blind!) field, NOT the CC field. Placing them into the BCC field will make the recipient not see all addresses. Has privacy and spam and virus advantages that way.
BTW, some servers will reject 50+ emails at once, so you might have to send 49 at one time. Some email clients will break up bulk emails into smaller "lots" automatically.