You need to use the Customization function. http://kb.mozillazine.org/Toolbar_customizing_%28Thunderbird%29

Thank you. I have already tried the customization function; I can see the hand in the dialog box with which to grab and drop, and I can grab but not drop. Help!

A 'Reply-all' button will auto display if the email you are viewing was sent to more than just you. If it was only sent to you, then there will be no 'Reply-all' because - well that's rather obvious :)

However, are you saying that even when the email was sent to several people including you, there is no 'Reply-all' button showing in the header area ?

Where are you trying to put the 'Reply-all' button? Are you talking about the 'Mail Toolbar' or the Header section in Message Pane which displays eg: 'Reply', 'Forward' > 'Archive' etc ?

When you select Customize for a particular toolbar that is the only place you can drop the new button. You do not get to choose any random place and that is why the NO symbol appears until you get it to the proper toolbar.

Many thanks for your answers; most useful. Another question if I may: on the individual mail page hoe can I get rid of all the gubbins below the message recipient, ie "Message ID" and the rest.

re : , ie "Message ID" and the rest.

Sounds like you are viewing the full headers. Do this to see the shorter version. 'View' > 'Headers' > 'Normal'