This is quick set of answers to the most common issues with extension development. They are currently written with mostly Firefox in mind, but most if not all should easily translate to SeaMonkey, Thunderbird or any of the other applications. For Thunderbird, you may also find the extension HowTo or FAQ pages helpful.
If you're looking for a place to get started, try our tutorial, Building an Extension or MozillaZine's Getting started tutorial. Use the Extension Wizard to generate a template to start with.
Be sure to set the development preferences.
TODO: Add content.
Where can I get more help?
Please see Extensions:Other Resources and Extensions:Community.
Before asking for help, be sure to set the debugging prefs and check the Error Console for related messages. Also, don't forget to do at least a simple web search before asking. And read this FAQ!