Unofficial Google Messages add-on for Thunderbird, it adds a button that opens a Google Messages tab in Thunderbird. Go to Thunderbird Add-ons page for download and reviews.
Open Thunderbird, go to Tools -> Add-ons -> Extensions, search for Google Messages in the search box and click on "+ Add to Thunderbird".
Download the repository, zip it, rename it to Google-Messages-Tab.xpi and choose install addon from file in Thunderbird.
In linux the xpi file can be created with the following commands
git clone https://github.com/sigurd-dev/Thunderbird-Google-Messages-Tab
cd ./Thunderbird-Google-Messages-Tab
VERSION=$(cat ./manifest.json | jq --raw-output '.version')
zip -r "../Google-Messages-Tab-${VERSION}-tb.xpi" *
This is really just a "rip off" of the Google Keep Tab where links and icon were changed.