I guess it's entirely on the table Thunderbird is, in fact, becoming an Electron app. Thunderbird the project has no ties financially to Mozilla anymore AIU, they're merely using gecko as embedded browser. From the comments in this thread, I get that Thunderbird is technically a hold-out from the XUL times, or even a large old-Firefox plugin of sorts more so than a standalone app. With Moz themselves having abandoned their platform over ten years ago, why would the Thunderbird team venture into creating an Electron-workalike based on Firefox when even Moz don't do so? Doesn't sound like something you'd want your mail client team to do considering initial and ongoing effort either (nor strategically, I'm afraid to add, given Moz's trajectory).

Seems odd to me to describe using a Chromium-based toolkit as a way to break the Chrome monoculture. Yeah, I know it is open source, but the ties are still obvious.