At the risk of starting a my-programming-language-is-better-than-yours war 
… to the best of my knowledge, Java is memory safe. (There is no “free” at all in Java - so there are no use-after-free errors, no double-free errors - and all memory is type safe - and all array accesses are checked … . So large classes of programming errors are simply not possible, as compared with languages like C.)
If you look more at the report it seems like what they mean is: They have changed Thunderbird so that Rust code can be integrated within Thunderbird and they did that in order to have safe support for Microsoft Exchange-related protocols. But I don’t really know the details (and Exchange support is not needed by me).