There are two sides to this migration. The main Firefox UI is being rewritten (how much of XUL remains after this rewrite is unclear), and extensions developed in XUL are being dropped.
https://www.neowin.net/news/mozilla-is-working-on-a-new-ui-f...
https://blog.mozilla.org/addons/2017/02/16/the-road-to-firef...
This deprecation of XUL extensions could prove problematic for Thunderbird, as it means that Mozilla can now move a lot faster and make breaking changes without having to worry about mass-breakages in extensions, therefore making it more likely that they'll leave Thunderbird behind at some point, but XUL itself isn't yet being deprecated.
There are some long-long-term plans to maybe eventually replace XUL with HTML/CSS/JS, but they need much better performance still before that becomes viable. There are some efforts into that direction already[0], building a framework for it as well, and those mostly focus around Servo, as that will have the necessary performance, so I'd guess at least a couple of years still.