I'd try turning it onto its head; select the stuff you definitely DO want to forward, then copy and paste into a new message. Or, and this may be counterintuitive, select the part you're interested in and hit Reply To Sender. It will copy just the selected text into a new message, leaving you just the task of removing that one address from the addressing box. I'd be wary of this approach, as it will carry a link back to the previous message. Under some circumstances, this can lead to apparently unrelated messages being added to a thread. Unfortunately there's no easy or automatic way for a program such as Thunderbird to know which bits you do and don't want to include in your forwarded message.