It's not just the "rendering engine" it's the entire software stack from down at the bottom with the IO and networking libraries to mid-layers of JS and core re

It's not just the "rendering engine" it's the entire software stack from down at the bottom with the IO and networking libraries to mid-layers of JS and core rendering, to very top with the UI toolkit. There's not much in Thunderbird that isn't Firefox and it's almost all custom Mozilla code, about 20 million lines of it, and not heavily reliant on major OS libs like so many other large-scale apps.