This has always been a great addon, does what it says, low overhead, no complaints.
It was actually created to fill a gap that was created in Thunderbird's development: you used to be able to minimize the headers directly from the program, then that useful feature was removed from one release to the next. Why? Are M$ employees working on TB? Seriously, did a group have a meeting wherein the topic of removing usability and user-friendliness was discussed and forcing unwitting users to sacrifice screen real-estate because of a perceived need for 'code elegance' or 'compactness' took hold?
Meanwhile, thanks to the developer of this extension, jmozmoz, compacting the headers to a single relevant line was the job of simply clicking one button and the state was remembered well between program invocations or computer reboots, whether Linux or Windburps. Now, a program update, apparently a minor revision [?] has left this extension out in the rain (v. 2.10 TB) works no more in TB 31.8.0 [alas].
Compared to the unrest in the world, this is only a hiccup, or possibly a small hiccaburp, but would leave the world an even politer place by somehow being fixed.