Rust is wildly fast. Learning that it is being used for a program is good to know if you care about speed. If you read the article, it even addresses your exact critiques:
Moreover, Rust has demonstrated superior performance compared to JavaScript add-ons, resulting in a quicker and more responsive Thunderbird. Furthermore, the integration of Rust into Thunderbird will be facilitated by the fact that it is already utilized in Firefox, enabling Thunderbird to leverage existing infrastructure for testing and continuous integration.
So not only with thunderbird be faster because Rust is faster than JavaScript, but it eliminates 3rd party addons by being native which also further increases speed. Lastly, development time for new features and improvements is faster because they can now use using the mature tooling that Mozilla has for Rust.