Yes Addons for developping Addons !

These are three firefox extensions recommended for anyone wanting to develop extensions:

• Extension developper, seems this one has a problem but the link goes to the mozilla page that lists other extensions
• Consoles² in an enhanced javascript debugger console, it will display javascript error and will be helpful for debugging your javascript snippets or programs
• Javascript Shell si very handy to make quick javascript snippet tests

I recommend you to master javascript if you want to make the most of a webpage, because it is the only client side script language that allow nice effect and stuffs, and you'd be very glad to develop your very own firefox extensions.