Review for Quicktext by James W
Rated 4 out of 5 stars
I was worried when reading about this add-on no longer working, so looked for an alternative.
This seems to do what I need and is compatible with the latest Thunderbird:
https://addons.mozilla.org/en-us/thunderbird/addon/clippings/
To create your own collections, you must have a Mozilla Add-ons account.