Can anyone confirm whether or not it's built on Electron?
The closest I can find is from Feb 2023, saying that at that time it would not be Electron:
Mozilla also still plans to use the Firefox web browser as the core platform for Thunderbird. That leaves Thunderbird as one of the few cross-platform mail applications that isn’t an Electron app or based on web technologies in some way
So possibly Electron-like but based on Firefox rather than Chromium?