From what I could tell, the early versions of Jetpack supported developing plugins for both Firefox and Thunderbird, but the current site only mentions Firefox 4.
Did they drop support for Thunderbird?
If not, can someone point me to a hello world style example? Google has not been helpful. :/
Most likely some, possibly many, parts of the SDK will still work, just as they used to, but you're likely to face bugs that you'll have to figure out mostly on your own.
The core team never supported or tested the use of SDK to build Thunderbird add-ons, which is why the declaration of Thunderbird support was removed from install.rdf.
My guess is that you just have to undo that change to install.rdf to get started.
Here's a post from someone else using the SDK with Thunderbird.
