Two Thunderbird developers have tried to implement new task kinds outside of mozilla- code in the past couple of months with very limited success. These tasks fall into one of three categories:

- a task kind exists in Thunderbird, but the definitions are direct references to the Firefox definitions. These are underlying components of the build system: docker images, debian package rebuilds, fetch task, toolchain builds
- does not clearly map to an existing task kind
- an existing task kind makes sense, but the schema is limited in a way that does not work for Thunderbird.

Specific use cases that have come up:
- Bug 1574835 - gdata-provider multilang repack at buildtime
  - This was abandoned for other reasons, but would have been implemented as a separate task and likely would have needed a new task kind
- Bug 1573570 - The requirement was to download an external file and include it in the build. This would be implemented as a fetch task. As mentioned above those are all imported from Firefox and there is no existing way to define individual tasks of a single kind in multiple trees
- Bug 1518166 - A third party library needs to be added to the build. The code is vendored in the repository, but is quite finicky on OS's other than Linux and pretty much demands that it be cross-compiled when targeting Windows. Implementing as a toolchain build seems to be the obvious solution, however limitations on the schema and script paths have made this a challenge. It could be implemented as a separate build task, but there are similar problems. Ideally the cached_task transform would be applied like for toolchain builds, but transforms apply to every task in a kind. This can be worked around, but this would just work better as a toolchain task.
- Bug 1586508 - It comes down to workflow differences between how Firefox and Thunderbird operate. Throw in Seamonkey and the existing strategies don't quite cover what we'd like.