First off make sure you've read Firebug's docs. Some of the commands work cross-brower with other tools as well.
A simple search query will show you all available extensions for Firebug. As some people mentioned - some of them are really helpful.
Also it's important not to limit yourself to just a single tool since you will most likely be developing for multiple browsers. So make sure you take a look at webkits developer tools (Safari, Chrome) as well. Here's a good article which sums up the most popular development/debug tools.
You might want to research how jQuery/jQuery plugins are structured/organized so you have general idea how to organise your own JavaScript/jQuery code. It all depends how JavaScript heavy is your application. If jQuery just provides some visual enhancements and few Ajaxified pages here and there, don't bother. From other hand if it's very JavaScript heavy (as in a lot more site logic on client-side then on backend) I would suggest Prototype over jQuery, but it's just my opinion.
You could consider using automatic tools to build your JavaScript if you have a lot of code.
For example:
On production server you want to end up with as few JavaScript files as possible and make sure to compress em.
If you're interested in more links to articles/tools for javascript heavy applications, drop a comment. I'm just trying to stay on topic at the moment.