I think a lot of people are forgetting that there is still a community developing it since Mozilla stopped "updating" it, beyond maintenance, in 2012. The product isn't dying, just will no longer be Mozilla backed. I assume that maintenance, like ongoing updates, will be passed to the community and hopefully another company steps up to back the project.
No problem. I think everyone got out their pitchforks without reading Mozilla's announcement and looking at the repo.
There are always issues to help out on. Here's a bit more info on contributing [0]. I imagine it won't change much as the project is officially handed off to the community. It's largely led by the community anyways at this point.
Repo for Thunderbird: https://hg.mozilla.org/comm-central