The first step towards a multi-process architecture has been made in Thunderbird Daily. The current implementation won't provide "noticeable improvements" for the foreseeable future, but it is the cornerstone that future improvements will build-on.
Thunderbird users won't notice changes when they upgrade to a Daily version that supports multiple-processes. Users may check the system's process manager to verify that Thunderbird uses multiple processes, but that is about it currently.
Thunderbird loads extensions in child processes, and will use processes for content that is loaded, e.g. the homepage or a new tab page or window.
Beta versions of Thunderbird may receive the update as early as next week, and Thunderbird Stable later in 2021, likely after the release of the next Thunderbird ESR version, version 91 in Summer.
www.thunderbird.net
www.ghacks.net
www.mozilla.org
