Needed Thunderbird Extension APIs

This is a wishlish page where AMO editors or extension authors can add areas that are *both* 1) extended often and 2) very hard to extend by pure JS extensions. Please only add areas that qualify for both, not just 2).

You don't need to suggest a concrete API. More important is to describe what you wanted to do (high-level, user perspective) and why the current API was a pain.