First off, Firefox is a minimalistic browser and does not come with many built-in web development features. Despite that, it has plenty of features that are really useful compared to their counterparts in other browsers (cough.. IE).
To name a few:
- Firefox has a really nifty source code viewer with code highlighting and many other bells and whistles (right click > View Page Source)
- an error console
- and a page information tool that lists detailed information about a page (right click > View Page Info)
I am sure there are some more that I have not yet discovered.