Still works, sort of Rated 3 out of 5 stars

CompactHeader does work in Thunderbird 12, but only partially. It narrows the message header pane as it's designed to do. However, the icons to delete, reply, etc. can not be installed in compact view, and the options to position the header at the left or right do not function.

This review is for a previous version of the add-on (2.0.2).  This user has other reviews of this add-on.

Could you please test if version 2.0.4beta2 (in the development channel fixes these problems)