There are a lot of Firefox Extensions out there who can support you in your website development and design process. The following three are those I find the most useful.

• Web Developer (All sorts of page information and on-the-fly modification possibilities)
• HTML Validator (Quick check for validity of your Code)
• Stylish (Manage and apply different stylesheets to existing webpages)

I am using these three Firefox-Extensions frequently for my everyday work and would not want to work without them anymore.