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 0.8.2 of CompactHeader

Or go to addons.mozilla.org