I’ve made a basic Hubspot for Thunderbird extension here.
It allows you to add contacts to Hubspot from Thunderbird and to see a contact’s history in Thunderbird.
It’s not on the extensions store yet, just on Github, but it’s a signed .xpi file. Look in the Releases section on github.
Note, it does not do tracking. There’s no tracking pixel. From initial reading, that’d need an entirely different approach.
Anyway, comments welcome - here or Github.