Yes, this is why the issue has become urgent right now. Thunderbird barely has the resources to keep up with the underlying changes to Gecko with each release. It has nowhere near the resources needed to deal with the deprecation of XUL. I am grateful to the Thunderbird community for continuing to maintain a stable email client over the years, but it has been a long time since a new release has brought any new feature that impacted me. From following bugs on some features I would like to see, I get the impression that the code base is old and full of overly intertwined components that make changes difficult. One example is this bug opened eight years ago about making it possible to write emails in a new tab rather than a separate window: https://bugzil.la/449299

I hope that once the Thunderbird code base is split from Firefox the churn for the developers will be reduced and they can spend more time on improving performance and adding new features. I do worry though that Thunderbird could become less secure over time without the Firefox team maintain security issues in XUL.

I find it hard to believe that another organization would be a better home for Thunderbird than Mozilla, but some of the language in the blog post and report is disappointing to me. Mainly, the points about Thunderbird needing to be self-funding and use its own infrastructure. If the code bases split and Mozilla focuses its advocacy around Firefox, surely that should be enough to unencumber Firefox and give it a (in my opinion slim) chance to remain relevant. Mozilla really values Thunderbird so little that it can't give it any resources at all? Even when it has all of the infrastructure needed by Thunderbird already set up?