It is not possible to switch between collapsed and expanded message headers without using the mouse. This is a bad point concerning accessability. I am talking about the little plus in the standard them which is in the message part of the main window, right next to the message subject. So not the message listing, but the frame in which the message itself is shown. For more details and a discussion, see Usenet message egnlrm$ahf$1, in alt.fan.mozilla. There is an extension which almost offers this functionality, Headers Toggle (https://addons.mozilla.org/thunderbird/1489/). However, it does a three-way toggle: normal, all and collapsed headers. Switching between normal and all headers is already possible through View -> Headers -> All or Normal, which can be reached over Alt+V, then h, then a or n. There is no similar switch, or menu item, or shortcut to expand/collapse the message header.

OS=ALL? valid accessibility issue?

assuming you can focus on the element (which I believe you can) then you can activate it; so it's not completely inaccessible. I'm fairly sure this has gone away w/ TB3 since we currently have a ( show details ) button and with bug 480623 we'll only have the detailed header view.

duping to bug 480623. please reopen the bug if you find your issue is not resolved by version 3