Unofficial WhatsApp Web add-on for Thunderbird, it adds a button that opens a WhatsApp Web tab in Thunderbird.
You can use this add-on instead, which provides the same functionality and will presumably get updated.
Download the repository, zip it, rename it to WhatsTab.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/lemtom/Thunderbird-WhatsTab
cd ./Thunderbird-WhatsTab
VERSION=$(cat ./manifest.json | jq --raw-output '.version')
zip -r "../WhatsTab-${VERSION}-tb.xpi" *