Originally Posted by randomizer
It is definitely supported but not the same way that it used to be. Thunderbird is no longer managed by Mozilla, nor does it use any of Mozilla's infrastructure.
True, although Mozilla is still serving as the legal and fiscal home for the project and the site still has Mozilla mentioned and linked to at the bottom, so they are still involved in some areas. For example, regarding the fiscal home part, if you want to donate to the project, Mozilla collects the money for them:
While Thunderbird is now an independent project separate from Mozilla, Mozilla has agreed to collect donations on our behalf.
And I'm sure that they won't be denied some technical assistance / clarification regarding Firefox's source code when they need it to include in Thunderbird.
But yes, things did change last year. But to be honest, Thunderbird doesn't need to be updated at the same pace as Firefox does, so what they did makes sense. They are a bit behind in adopting WebExtensions though, and still using the old style extensions, that is true. Although I feel extensions are more security critical in a web browser due to the nature of web pages; an e-mail application can wait a bit more if you set it right from a security perspective, although of course your mileage will vary, but I don't feel the need for extensions myself on an e-mail client.