Therefore I believe Thunderbird should would thrive best by separating itself from reliance on Mozilla development systems and in some cases, Mozilla technology. The current setting isn't stable, and we should start actively looking into how we can transition in an orderly way to a future where Thunderbird and Firefox are un-coupled. I don't know what this will look like, or how it will work yet. I do know that it needs to happen, for both Firefox and Thunderbird's sake. This is a big job, and may require expertise that the Thunderbird team doesn't yet have. Mozilla can provide various forms of assistance to the Thunderbird team via a set of the Mozilla Foundation’s capabilities.
Are there still any Thunderbird users left? It's been in maintenance mode for a while, and there's several great alternatives (some of them even based on Thunderbird). That being said, having Thunderbird as a separate entity from Firefox, that can make its own decisions, could benefit the open source project greatly.