Thunderbird does not anymore have the ability to reduce the display size of header planel to one line starting with beta 3 (see mozilla wiki, release notes and bugzilla).
This addon adds a symbol to compact and expand the message header plane. Beside this, it allows whether the buttons added to Thunderbird 3.0 in the header plane should be displayed or not.
There is a thread on mozillazine.org to discuss this addon.
You can also download it from addons.mozilla.org