I hope this service will use JMAP and push the Thunderbird client itself to adopt it
If the article is correct, Thundermail will be built using Stalwart[1], which appears to support JMAP
[1]: https://stalw.art/
Speaking about thunderbird, I liked their UI redesign, but it seems they are taking away quite a bit of plugin capabilities, e.g. there used to be the possibility to run firenvim (a plugin to run neovim in the compose window), but that's not possible anymore.
Previously: