What are some resources for getting started writing a Firefox Addon? Is there an API guide somewhere? Is there a getting started tutorial somewhere? Is there a developer discussion board somewhere?
We tried to make https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions answer all those questions. The first three links in the documentation section are about getting started (that includes something like Adam's link, before it became stale). The newsgroup and the irc channel in the Community section are the official discussion boards.
Mozilla is very complex, so any kind of API guide would be overwhelming and hard to write. So your best bet is to check the code snippets page (also linked from the MDC Extensions page), then search MDC/google, then ask in the forums.