I can't think why you are not offered a Reply All button. I have one in my Thunderbird. (But mine is in the main toolbar, not the toolbar attached to the message itself.)

Does a right-click on the message offer you a Reply All option?

Right clicking didn't give me any options. I did just add the reply, reply all and forward to the main toolbar but it seems strange that the message toolbar will include reply and forward but not reply all.

Reply List and Reply all are dynamically present when the message addressing indicates it is relevant.