Unofficial Google Keep extension for Thunderbird, it adds a button that opens a Google Keep tab in Thunderbird. The home page of the extension contains some pictures and reviews.
Simply search for ThunderKeepPlus inside Thunderbird and the addon should show up.
Download the repository, zip it, rename it to ThunderKeepPlus.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/Garoe/ThunderKeepPlus
cd ./ThunderKeepPlus
VERSION=$(cat ./manifest.json | jq --raw-output '.version')
zip -r "../ThunderKeepPlus-${VERSION}-tb.xpi" *