I was searching for an Add-On to Thunderbird v24 that would batch-add all the message’s recipients to a new Contact Group.
Gina at LifeHacker pointed out a good one. And the developer’s site is here: http://addresscontext.mozdev.org/index.html
But it is incompatible with the latest version of Thunderbird. Please update it, Mike.
If you need this too, email the author:
mikekrieger [at] gmail [dot] com