As far as I can remember (I've been using Thunderbird for the past few years) the reply will go only to the forwarder who is after all the person sending the forwarded email. To send an email to the originator you probably need to right click on that person's name in the forwarded email and there is probably an option to compose an email to that person.
PS - forgot to mention that I don't use OE, so just want to add that in order to change the To: address, the person you forwarded it to may need to first move the email to the Drafts Folder, make the change of address and then Send direct from the Drafts Folder.
Reply All won't do what you want on a forwarded email as the only address it can reply to is yours (the sender), unless you have forwarded to the originator as well. Otherwise Technotiger's posts have the answer.