Installation
Download the xpi file from the link below by placing the mouse over the link and start the download by the context menu.

For Thunderbird versions before b4pre of 09/09/10 use version 0.6.2:

Version 0.6.2 of CompactHeader

For Thunderbird versions starting with b4pre of 09/09/10 use version 0.7.0 or newer:

Version 0.7.1 of CompactHeader

Or go to addons.mozilla.org