Add spamscore header to Thunderbird
As of 2019-02-10 only this plugin / approach seems to work:
"rspamd-spamness" V0.9+
https://github.com/moisseev/rspamd-spamness
Note: currently V0.9 is not on Thunderbirds Add-Ons site!
Manual Installation
You don't have to wait for release. An XPI file is simply a ZIP file, containing the add-on files. You can just download source files from GitHub and repack them so they are located at the root of the ZIP
Then you can rename it to .xpi or just drag and drop as is to `Add-ons Manager` tab.
Below attached is such a created .xpi V0.9