Two I can think of: - Performance. Rendering emails, especially big HTML emails, in a separate process means that rendering can be offloaded to a different CP

Two I can think of: - Performance. Rendering emails, especially big HTML emails, in a separate process means that rendering can be offloaded to a different CPU core and that can help keep the main Thunderbird UI more responsive - Security. If HTML email rendering is in its own process it reduces the blast radius of any exploits in the web sandbox. I also vaguely recall from Firefox's work that there might be additional OS-level security restrictions that can be applied on a per-process basis, which you might want for web rendering but can't have for other parts of the email client.