ExQuilla is a paid addon and requires a paid license. Keeping ExQuilla working with future Thunderbird versions is a lot of work and ongoing.
ExQuilla was originally developed by R Kent James. Due to retirement, ownership and responsibility has been taken over by Beonex GmbH. Beonex has been contributing a lot of Open-Source software to Thunderbird over 20 years. But we also need to live, so this paid addon allows us to pay our rent.
The new homepage is https://www.exquilla.com . Other websites might be outdated.