Mozilla does allow users to develop Firefox extensions with Jetpack, however if you are looking for something more detailed, you can download a free e-book (limited time only) that will help you get kick started in developing add-ons/extensions for Firefox.
The book is written by James Edwards and usually retails for $9.95, however for a limited time you can download it for absolutely free. Thanks Sandip.
The book covers several topics including;
- Learning about folder structures and files in a extensions
- Developing user interface with XUL and styling with CSS
- Keyboard shortcuts and I18N features
- Guide to publish your add-ons to Mozilla add-ons gallery