The most practical, logical, obvious move would to be to reimplement Thunderbird as an Electron app.
But of course that would be untenable for Mozilla because it would be admitting defeat to Chrome.
The problem is that Mozilla has no alternative to Electron. There are a number of false starts and head fakes, but none of them have any buy in or support or future.
What would it take for Mozilla to build an Electron-like runtime with Firefox?
They tried:
"This project is an Electron-compatible app shell for creating desktop apps based on Gecko, the rendering engine used in Firefox."
https://github.com/mozilla/positron
but they failed: "As noted in the blog post Positron Discontinued [1], this project has been discontinued. The source remains available, and you're welcome to reuse it." [1] https://mykzilla.org/2017/03/08/positron-discontinued/